Lightroom Classic: Cannot delete Collections, or move images between them

First time trying to do new work using LR 7.2 since upgrade, previously it has rebooted my PC when running the Duplicates II plugin (which had run successfully just before the upgrade ...!). I do NOT like the thought that things can get so messed up that I get a reboot, especially on a system that usually stays running 24/7!

Anyway - the problem: When in the Library, went to Collections and decided I would simplify a collection that tracks what images I've printed, from numbered sub-collections to simple Printed / Not yet Printed collections (I'm tracking the numbers/sizes etc. in a spreadsheet now). However, when I tried to delete some empty catalogs they didn't disappear, whether I use the right-click & delete or the hyphen at the top of the collections area.

Then, I found I couldn't copy images between collections. I haven't delved further yet, but this is the first time something like this has happened for me since I started with the LR 1 beta.