I'm facing the same problem with the AppCenter, the updates page is blank nothing is shown, none of the apps installed, neither updates.
I usually update the system through AppCenter, I remember that this problem began to occur after I ran the sudo apt autoremove command that theoretically would remove packages that are no longer used, but it seems that
Removed more than it should. Older kernels were removed and with that the Nvidia driver stopped working, I had to reinstall it. I can update the system normally from the command line, no problem.
To install applications I have to use the command line also since in App Center it just keeps waiting ... and does not install the application,
sudo apt list --upgradable normally lists packages that can be updated
What I've done to try to fix the problem:
sudo apt-get purge appcenter
sudo apt-get purge packagekit
3 - Restart, reinstall, unsuccessful
4 - I removed any other software source I could find
5 - Restart, unsuccessful
6 - Follow the procedures on the links:
No software is shown in AppCenter except what is installed
Loki AppCentre updates page blank, can't do 1 update
I can't find FireFox in the AppCenter of Loki, although I have installed it
Also unsuccessfully, do you have any idea how to fix it?