I have just upgraded OwnCloud from version 10.2 to version 10.3 and have run the occ upgrade command without any problems.
I use IMAP to authenticate my users and after the upgrade I have a lot of “User backend OC_User_IMAP not found” errors. Nothing has changed in the config.php file.
The user_external app is enabled and I have tried to disable it and then enable it again, without any success
if such issues appears after an upgrade i would personally report these to a bugtracker rather then to a user based support forum. It seems the bugtracker for that specific app is available at: