Negative numbers for UIDs?

[Christof Baumgärtner <offlineimap@xxxxxxxx>]

Hi,
I am synching between two IMAP servers. It seems that every time nearly all 
messages are copied from one server to the other. Is this related to one of 
the servers not supporting UIDs properly? I know for sure that the one 
server does not support IMAP SEARCH and IMAP IDLE.
The maybe interesting part is that the message numbers are negative numbers:
e.g.
Copy message -65 MappedIMAP[INBOX] -> IMAP[INBOX], LocalStatus[INBOX]
Copy message -46 MappedIMAP[INBOX] -> IMAP[INBOX], LocalStatus[INBOX]
Copy message -67 MappedIMAP[INBOX] -> IMAP[INBOX], LocalStatus[INBOX]
Copy message -66 MappedIMAP[INBOX] -> IMAP[INBOX], LocalStatus[INBOX]
Copy message -11 MappedIMAP[INBOX] -> IMAP[INBOX], LocalStatus[INBOX]
Copy message -64 MappedIMAP[INBOX] -> IMAP[INBOX], LocalStatus[INBOX]
Copy message -63 MappedIMAP[INBOX] -> IMAP[INBOX], LocalStatus[INBOX]
Copy message -62 MappedIMAP[INBOX] -> IMAP[INBOX], LocalStatus[INBOX]
Copy message -61 MappedIMAP[INBOX] -> IMAP[INBOX], LocalStatus[INBOX]
Copy message -60 MappedIMAP[INBOX] -> IMAP[INBOX], LocalStatus[INBOX]
snip

Runing the synch again generates one additional copy per message per sync.

How can I track down this problem?
Thanks for your help.

Best regards,
Christof