If one of the users in our group tries to login to his desktop computer, he gets a popup "Authentication required to refresh system repositories". When I type in my administrator password in this popup, the popup does not disappear. It seems that only a hard reboot solves the problem temporarily, because after a few days the same popup appears again. When I login as a user with sudo privileges, though, everything works fine. The operating system is Ubuntu 19.10.
The user has no scripts running on startup, other than loading his bashrc and setting his environment variables.
I already searched the internet, but I have been unable to find a solution (I have done the most recent upgrades). I would really appreciate your help.