Checking apt in the terminal revealed the problem: Probably connected to Spectre and Meltdown patches, some graphic driver upgrades seemed to cause dependency problems.
$ sudo apt update && sudo apt upgrade
[...]
Reading package lists... Done
Building dependency tree
Reading state information... Done
You might want to run 'apt-get -f install' to correct these.
The following packages have unmet dependencies:
libdrm-dev : Depends: libdrm-amdgpu1 (= 2.4.89+git1801151830.57d3d4~oibaf~x) but 2.4.83-1~16.04.1 is installed
libgl1-mesa-dri : Depends: libdrm-amdgpu1 (>= 2.4.89+git1801151830.57d3d4~oibaf~x) but 2.4.83-1~16.04.1 is installed
libgl1-mesa-dri:i386 : Depends: libdrm-amdgpu1:i386 (>= 2.4.89+git1801151830.57d3d4~oibaf~x) but 2.4.83-1~16.04.1 is installed
mesa-va-drivers : Depends: libdrm-amdgpu1 (>= 2.4.89+git1801151830.57d3d4~oibaf~x) but 2.4.83-1~16.04.1 is installed
mesa-vdpau-drivers : Depends: libdrm-amdgpu1 (>= 2.4.89+git1801151830.57d3d4~oibaf~x) but 2.4.83-1~16.04.1 is installed
E: Unmet dependencies. Try using -f.
$ sudo apt install -f
Reading package lists... Done
Building dependency tree
Reading state information... Done
Correcting dependencies... Done
The following packages were automatically installed and are no longer required:
geany-common libdrm-common libllvm4.0 libllvm4.0:i386 libxslt1-dev linux-headers-4.4.0-101
linux-headers-4.4.0-101-lowlatency linux-headers-4.4.0-103 linux-headers-4.4.0-103-lowlatency linux-headers-4.4.0-104
linux-headers-4.4.0-104-lowlatency linux-image-4.4.0-101-lowlatency linux-image-4.4.0-103-lowlatency
linux-image-4.4.0-104-lowlatency mod-host mod-ui mod-ui-common phantomjs python-lockfile python3-pystache python3-serial
python3-tornado
Use 'sudo apt autoremove' to remove them.
The following additional packages will be installed:
libdrm-amdgpu1 libdrm-amdgpu1:i386
The following packages will be upgraded:
libdrm-amdgpu1 libdrm-amdgpu1:i386
2 upgraded, 0 newly installed, 0 to remove and 1 not upgraded.
41 not fully installed or removed.
Need to get 0 B/60,3 kB of archives.
After this operation, 20,5 kB of additional disk space will be used.
Do you want to continue? [Y/n] y
(Reading database ... 656858 files and directories currently installed.)
Preparing to unpack .../libdrm-amdgpu1_2.4.89+git1801151830.57d3d4~oibaf~x_amd64.deb ...
De-configuring libdrm-amdgpu1:i386 (2.4.83-1~16.04.1) ...
Unpacking libdrm-amdgpu1:amd64 (2.4.89+git1801151830.57d3d4~oibaf~x) over (2.4.83-1~16.04.1) ...
dpkg: error processing archive /var/cache/apt/archives/libdrm-amdgpu1_2.4.89+git1801151830.57d3d4~oibaf~x_amd64.deb (--unpack):
trying to overwrite '/usr/share/libdrm/amdgpu.ids', which is also in package libdrm-common 2.4.83-1~16.04.1
Preparing to unpack .../libdrm-amdgpu1_2.4.89+git1801151830.57d3d4~oibaf~x_i386.deb ...
De-configuring libdrm-amdgpu1:amd64 (2.4.83-1~16.04.1) ...
Unpacking libdrm-amdgpu1:i386 (2.4.89+git1801151830.57d3d4~oibaf~x) over (2.4.83-1~16.04.1) ...
dpkg: error processing archive /var/cache/apt/archives/libdrm-amdgpu1_2.4.89+git1801151830.57d3d4~oibaf~x_i386.deb (--unpack):
trying to overwrite '/usr/share/libdrm/amdgpu.ids', which is also in package libdrm-common 2.4.83-1~16.04.1
Processing triggers for libc-bin (2.23-0ubuntu9) ...
Errors were encountered while processing:
/var/cache/apt/archives/libdrm-amdgpu1_2.4.89+git1801151830.57d3d4~oibaf~x_amd64.deb
/var/cache/apt/archives/libdrm-amdgpu1_2.4.89+git1801151830.57d3d4~oibaf~x_i386.deb
E: Sub-process /usr/bin/dpkg returned an error code (1)
After googling a bit on how to reverse such an unsuccessful ugrade, I came across this article which gave me enough info to adopt for my problem. $ grep -A 3 'Start-Date: 2018-01-17' /var/log/apt/history.log revealed one big chunk of text:
[...]
--
Start-Date: 2018-01-17 10:05:28
Commandline: packagekit role='update-packages'
Requested-By: peter (1000)
Upgrade: libgles2-mesa:amd64 (17.2.4-0ubuntu1~16.04.2, 17.4~git1801160730.1cbcd7~oibaf~x), uget-chrome-wrapper:amd64 (2.0.6-1, 2.0.7-1), libdrm-nouveau2:amd64 (2.4.83-1~16.04.1, 2.4.89+git1801151830.57d3d4~oibaf~x), libdrm-nouveau2:i386 (2.4.83-1~16.04.1, 2.4.89+git1801151830.57d3d4~oibaf~x), gir1.2-gdkpixbuf-2.0:amd64 (2.32.2-1ubuntu1.3, 2.32.2-1ubuntu1.4), libegl1-mesa-dev:amd64 (17.2.4-0ubuntu1~16.04.2, 17.4~git1801160730.1cbcd7~oibaf~x), libgdk-pixbuf2.0-0:amd64 (2.32.2-1ubuntu1.3, 2.32.2-1ubuntu1.4), libglapi-mesa:amd64 (17.2.4-0ubuntu1~16.04.2, 17.4~git1801160730.1cbcd7~oibaf~x), libglapi-mesa:i386 (17.2.4-0ubuntu1~16.04.2, 17.4~git1801160730.1cbcd7~oibaf~x), mesa-common-dev:amd64 (17.2.4-0ubuntu1~16.04.2, 17.4~git1801160730.1cbcd7~oibaf~x), libegl1-mesa:amd64 (17.2.4-0ubuntu1~16.04.2, 17.4~git1801160730.1cbcd7~oibaf~x), carla-git:amd64 (2:1.9.7+git20180106, 2:1.9.8+git20180116), libgbm1:amd64 (17.2.4-0ubuntu1~16.04.2, 17.4~git1801160730.1cbcd7~oibaf~x), carla-lv2:amd64 (2:1.9.7+git20180106, 2:1.9.8+git20180116), libdrm-amdgpu1:amd64 (2.4.83-1~16.04.1, 2.4.89+git1801151830.57d3d4~oibaf~x), libdrm-amdgpu1:i386 (2.4.83-1~16.04.1, 2.4.89+git1801151830.57d3d4~oibaf~x), libllvm5.0:amd64 (1:5.0-3~16.04.1, 1:5.0.1-1~oibaf~x), libllvm5.0:i386 (1:5.0-3~16.04.1, 1:5.0.1-1~oibaf~x), xserver-xorg-video-intel:amd64 (2:2.99.917+git1706071933.6babcf~gd~x, 2:2.99.917+git1801120736.26f540~oibaf~x), libwayland-egl1-mesa:amd64 (17.2.4-0ubuntu1~16.04.2, 17.4~git1801160730.1cbcd7~oibaf~x), carla-data:amd64 (2:1.9.7+git20180106, 2:1.9.8+git20180116), libgles2-mesa-dev:amd64 (17.2.4-0ubuntu1~16.04.2, 17.4~git1801160730.1cbcd7~oibaf~x), distro-info-data:amd64 (0.28ubuntu0.6, 0.28ubuntu0.7), libdrm2:amd64 (2.4.83-1~16.04.1, 2.4.89+git1801151830.57d3d4~oibaf~x), libdrm2:i386 (2.4.83-1~16.04.1, 2.4.89+git1801151830.57d3d4~oibaf~x), carla-vst:amd64 (2:1.9.7+git20180106, 2:1.9.8+git20180116), libgl1-mesa-dev:amd64 (17.2.4-0ubuntu1~16.04.2, 17.4~git1801160730.1cbcd7~oibaf~x), libgl1-mesa-dri:amd64 (17.2.4-0ubuntu1~16.04.2, 17.4~git1801160730.1cbcd7~oibaf~x), libgl1-mesa-dri:i386 (17.2.4-0ubuntu1~16.04.2, 17.4~git1801160730.1cbcd7~oibaf~x), libgdk-pixbuf2.0-common:amd64 (2.32.2-1ubuntu1.3, 2.32.2-1ubuntu1.4), libgdk-pixbuf2.0-dev:amd64 (2.32.2-1ubuntu1.3, 2.32.2-1ubuntu1.4), libosmesa6:amd64 (17.2.4-0ubuntu1~16.04.2, 17.4~git1801160730.1cbcd7~oibaf~x), libosmesa6:i386 (17.2.4-0ubuntu1~16.04.2, 17.4~git1801160730.1cbcd7~oibaf~x), libgl1-mesa-glx:amd64 (17.2.4-0ubuntu1~16.04.2, 17.4~git1801160730.1cbcd7~oibaf~x), libgl1-mesa-glx:i386 (17.2.4-0ubuntu1~16.04.2, 17.4~git1801160730.1cbcd7~oibaf~x), libdrm-intel1:amd64 (2.4.83-1~16.04.1, 2.4.89+git1801151830.57d3d4~oibaf~x), libdrm-intel1:i386 (2.4.83-1~16.04.1, 2.4.89+git1801151830.57d3d4~oibaf~x), libgbm-dev:amd64 (17.2.4-0ubuntu1~16.04.2, 17.4~git1801160730.1cbcd7~oibaf~x), libdrm-radeon1:amd64 (2.4.83-1~16.04.1, 2.4.89+git1801151830.57d3d4~oibaf~x), libdrm-radeon1:i386 (2.4.83-1~16.04.1, 2.4.89+git1801151830.57d3d4~oibaf~x), mesa-vdpau-drivers:amd64 (17.2.4-0ubuntu1~16.04.2, 17.4~git1801160730.1cbcd7~oibaf~x), libdrm-dev:amd64 (2.4.83-1~16.04.1, 2.4.89+git1801151830.57d3d4~oibaf~x), mesa-va-drivers:amd64 (17.2.4-0ubuntu1~16.04.2, 17.4~git1801160730.1cbcd7~oibaf~x)
--
[...]
I didn't have previous knowledge of this, but it didn't take long to guess (and hope) that the brackets contained (original-version, upgrade-target-version). Then I just copied this into a text editor and batch edited this to fit the format package-name:arch=original-version package-[...], so I could easily copy-paste that back into the terminal to use with sudo apt install to downgrade to the versions that at least worked. But apt wasn't too happy with this and complained a bit, so with the goal of just getting something through I simply removed the packages from that list again of which it couldn't find the original versions somehow.
[...]
Reading state information... Done
E: Version '2.0.6-1' for 'uget-chrome-wrapper' was not found
E: Version '2.32.2-1ubuntu1.3' for 'gir1.2-gdkpixbuf-2.0' was not found
E: Version '2.32.2-1ubuntu1.3' for 'libgdk-pixbuf2.0-0' was not found
E: Version '2:1.9.7+git20180106' for 'carla-git' was not found
E: Version '2:1.9.7+git20180106' for 'carla-lv2' was not found
E: Version '2:2.99.917+git1706071933.6babcf~gd~x' for 'xserver-xorg-video-intel' was not found
E: Version '2:1.9.7+git20180106' for 'carla-data' was not found
E: Version '0.28ubuntu0.6' for 'distro-info-data' was not found
E: Version '2:1.9.7+git20180106' for 'carla-vst' was not found
E: Version '2.32.2-1ubuntu1.3' for 'libgdk-pixbuf2.0-common' was not found
E: Version '2.32.2-1ubuntu1.3' for 'libgdk-pixbuf2.0-dev' was not found
$ sudo apt install libgles2-mesa:[...]
[...]
E: Version '2:2.99.917+git1706071933.6babcf~gd~x' for 'xserver-xorg-video-intel' was not found
E: Version '0.28ubuntu0.6' for 'distro-info-data' was not found
...but finally, it let me through.
Reading state information... Done
libdrm-amdgpu1 is already the newest version (2.4.83-1~16.04.1).
libdrm-amdgpu1 set to manually installed.
libdrm-amdgpu1:i386 is already the newest version (2.4.83-1~16.04.1).
libdrm-amdgpu1:i386 set to manually installed.
The following packages were automatically installed and are no longer required:
geany-common libllvm4.0 libllvm4.0:i386 libomxil-bellagio-bin libomxil-bellagio0 libxslt1-dev linux-headers-4.4.0-101
linux-headers-4.4.0-101-lowlatency linux-headers-4.4.0-103 linux-headers-4.4.0-103-lowlatency linux-headers-4.4.0-104
linux-headers-4.4.0-104-lowlatency linux-image-4.4.0-101-lowlatency linux-image-4.4.0-103-lowlatency
linux-image-4.4.0-104-lowlatency mod-host mod-ui mod-ui-common phantomjs python-lockfile python3-pystache python3-serial
python3-tornado
Use 'sudo apt autoremove' to remove them.
Recommended packages:
libtxc-dxtn-s2tc | libtxc-dxtn-s2tc0 | libtxc-dxtn0
The following packages will be DOWNGRADED:
libdrm-dev libdrm-intel1 libdrm-intel1:i386 libdrm-nouveau2 libdrm-nouveau2:i386 libdrm-radeon1 libdrm-radeon1:i386 libdrm2
libdrm2:i386 libegl1-mesa libegl1-mesa-dev libgbm-dev libgbm1 libgl1-mesa-dev libgl1-mesa-dri libgl1-mesa-dri:i386
libgl1-mesa-glx libgl1-mesa-glx:i386 libglapi-mesa libglapi-mesa:i386 libgles2-mesa libgles2-mesa-dev libllvm5.0
libllvm5.0:i386 libosmesa6 libosmesa6:i386 libwayland-egl1-mesa mesa-common-dev mesa-va-drivers mesa-vdpau-drivers
0 upgraded, 0 newly installed, 30 downgraded, 0 to remove and 10 not upgraded.
41 not fully installed or removed.
Need to get 47,9 MB of archives.
After this operation, 59,2 MB disk space will be freed.
Do you want to continue? [Y/n]
Get:1 http://archive.ubuntu.com/ubuntu xenial-updates/main amd64 mesa-vdpau-drivers amd64 17.2.4-0ubuntu1~16.04.2 [1.718 kB]
[...]
Setting up xserver-xorg-video-intel (2:2.99.917+git1801120736.26f540~oibaf~x) ...
Processing triggers for bamfdaemon (0.5.3~bzr0+16.04.20160824-0ubuntu1) ...
Rebuilding /usr/share/applications/bamf-2.index...
Processing triggers for libc-bin (2.23-0ubuntu9) ...
$
And indeed, it worked! The GUI is back to normal, at least for now. I will be careful with upgrading for a while, but I wanted to at least share this.
