Visit an IMAP folder from a VM session 'A'. Visit the same IMAP folder from another VM session 'B' and make changes to it. If you come back to the session 'A' and do vm-imap-synchronize then you get the "folder buffer being entered from process" error.
An instance of this problem here:
Visit an IMAP folder from a VM session 'A'. Visit the same IMAP folder from another VM session 'B' and make changes to it. If you come back to the session 'A' and do vm-imap-synchronize then you get the "folder buffer being entered from process" error.