"Failed to request VPN secrets #3: No agents were available for this request"
I tried reinstalling openvpn and network manager, and tried different ovpn profiles and made sure I entered the correct login information...
Finally got the VPN working after finding the answer in the following link:
The network manager somehow uses nm-applet (which does not run by default) to access the VPN secrets.
You can add nm-applet in the System Settings > Applications > Startup by adding the custom command /usr/bin/nm-applet
I always install software-properties-gtk after installing Elementary OS and encounter this issue everytime with PPTP.
After reinstalling ElementaryOS the last time, I decided to skip installing software-properties-gtk and never encountered the issue since. Even after multiple restarts.
I think the software-properties-gtk package breaks the VPN configuration tool since I was able to connect to a VPN (PPTP) using a fresh live USB of Elementary OS without issues. The software-properties-gtk has been removed from Ubuntu 16.04 and might be causing issues with the newer packages of Ubuntu.