Just in case anyone runs across this thread later – we would never recommend deleting anything out of a library’s .data folder as Benpiggy suggested. It’s a creative solution, but removing things from the .data folder can sometimes cause unintended consquences.
Instead, for those who may experience this issue at some point, please make your first step to try recovering the library, as described here:
If that doesn’t help, please contact our technical support department for additional troubleshooting.