Was it somehow related to your database being imported from MM4?
Are you still able to reproduce it and could you shared your MM.DB for analyze (supposing you can still replicate the issue)?

In the end I had to delete MM5 and reinstall.

I'm unable to reproduce.

When I started MM5 this morning I got this message

Your library is being upgraded to the latest version. It can take up to several minutes...

Three hours later, it was still updating. When I canceled I believe it sent an error message. Cancelling left MM5 unusable.