ERROR: Backend could not find uid for message, returned 0

[Johnny]

When I run offlineimap I basically always get a message that 

,----
| imap: savemessage: returning new UID 0
| ERROR: Exceptions occurred during the run!
| ERROR: Backend could not find uid for message, returned 0
`----

If I repeat the command a few times, eventually all messages are
fetched? What is the reason for this message, as it seems bypassable?
Let me know any loginfo that may help...

Offlineimap 6.5.2.1

Thanks...
-- 
Johnny