About a month ago I updated to Linux Mint 21 from the link provided in Update Manager. I've since had problems with update manager, but today was the worst.

It suggested I update VueScan to the newest version 9.7.29 (462061b34f). Normally when I update anything, I get a prompt for my password, but this time it goes straight through to downloading a Flatpak file of about 11.2Mb. Once this finishes, it returns me to the Update Manager screen, suggesting I do the same thing again.

In between restarting the computer a couple of times, I must have tried this half a dozen times, to no avail. Does anyone have any idea what's causing this problem and how to fix it?

This article should help: https://itsfoss.com/update-flatpak/

Full disclosure: I have no personal experience with Flatpak.

Thank you so much. I used terminal option to update flatpak, and it worked, with the Update Manager now saying I'm up to date. It's a pity that Update Manager doesn't quite work (does it download the update but not register that it has?), but this workaround seems to solve it.