Hi, capjap. I had an issue with Reader Library where it just showed the menu bar at the top, no library window would load, had to force quit, etc. I did an uninstall/reinstall to fix it. Before you try that route, you might want to try this simpler solution first. See the link below:
https://www.mobileread.com/forums/showthread.php?t=59980
If you need to do an uninstall / reinstall, then there are some extra folders that you need to delete on the Mac. See the link below:
https://www.mobileread.com/forums/showthread.php?t=78921
Hope that helps. Good luck!