Hi there
Had exactly the same problem, solved it by chasing up on where groove agent wanted to put the vst content when installing, I did that by uninstalling and reinstalling. I also found that there were two lots of some of the files (searched for them with normal windows file explorer), so I deleted those from the second location (the one where GA did NOT put the files on installation) and then finally the other Agents appeared again in GA. It’s fine now but as you suggest,a right royal PITA
Best Regards
Dave