There seems to be a problem with new user directories and the imapc backend. On a test setup I get the following error in the log:

May 02 10:09:58 imap(user1@domain.com): Error: Couldn't create mailbox list lock /tmp/tmp0Y5TMn/dovecot1/users/user1@domain.com/imapc/mailboxes.lock: file_create_locked(/tmp/tmp0Y5TMn/dovecot1/users/user1@domain.com/imapc/mailboxes.lock) failed: safe_mkstemp(/tmp/tmp0Y5TMn/dovecot1/users/user1@domain.com/imapc/mailboxes.lock) failed: No such file or directory

The 'imapc' part in the user directory is not there (yet), and so the mailboxes.lock file cannot be created. A [SERVERBUG] Internal error occurred is returned to the client.
This may have already been broken in an earlier release.
Older releases don't seem to have the mailbox lock at this point, so they don't return an error when the imapc directory is not there yet.