... since there's no help from Zimbra support yet I digged into the sourcecode and checked out what was causing this error. Bug 43669 has been marked as a duplicate of Bug 43359. I checked this bug out and found a workaround.
The incorrect behaviour is triggered by Zimbra Message Store Blob compression not getting the correct message size to decide wether or not to compress it.
Since we do not know a date for 6.0.5 I tried to determine a workaround and finally found a solution to avoid this error:
Blob Compression is enabled by default for each message store and is been triggered by a treshold > 8kByte. So either disabling Blob compression or setting the blob compression treshold to a higher value is a temporary fix for this error.
Disabling blob compression can be done by
Do a zmvolume --list first to detect the correct values for --name and --id.
zmvolume --name message1 --id 1 --edit --compress false
It should be safe to enable / disable Blob compression if disk space is not a major concern. The setting is being applied in the fly so there is no need to restart servers or services.
At least this works for me. I hope this will help others too to get IMAP back to work until 6.0.5 is out.