I ran into this issue as well. I tried a couple of things - saving as new project and new set but didn’t have any luck. Then I started a new empty project and put my OT into USB mode, copied the bank files from the new, uncorrupted project, into my broken project. That fixed the issue.
I imagine this won’t be a good solution for folks who have actual data - sequences, samples, etc in all of their banks. But for whatever reason, in my case the corrupted/missing bank was empty so replacing it with new bank files worked.