>It shouldn't do this since it's set as default, but when i click ImageGlass and "Ok", the screen disappears and then
> reappears 2 seconds later, asking again which program i want to use. No matter how many times i select
>ImageGlass, it refuses to open.
ImageGlass doesn't have any "self-destruct" capability. If IG isn't in your "installed programs" list, then either a) it wasn't installed [i.e. portable version] or b) someone or something attempted to uninstall it. The latter could include a Windows 10 upgrade ...
Unfortunately, IG is not a "signed" app, which means it tends to set off alarm bells with WIndows security stuff. If you have anti-virus or other apps running, they may have tried to disable it. You say you're using it at work, do you have an IT department? If so, they may have also tried to uninstall it; you'll have to ask them.
IG is on your computer, if you say you can run it. So try setting a different app as the default, i.e Photos, open a few images, then set the default back to ImageGlass. [Maybe even logout and/or reboot in-between]. It might be necessary to physically hunt down where ImageGlass.exe is located, and browse for that file when setting the default [as the system no longer appears to "know" about Imageglass].
Kevin