That is not an 'Error" message: That is a normal Status message box.
What is NOT normal, is it is 'hung' at that point (from your description).
Is your Library on a LOCAL ATTACHED drive? Is / has that drive folder been excluded from cloud backup.
Calibre has to write files and make folders, make an entry into Metadata.db (the heart of Calibre) and maybe even fetch some metadata from the web.
Until that is done, that singing 'Fat Lady (message)", won't return to her seat (go away).
If the drive is local: then the usual culprit for interference is you Anti-Virus-Security software. The other one is failing drive: Run the disk check utility for your OS
If you put the Library directly into the cloud: the usual culprit is the users failure to read the FAQ
warnings.