Hi,
There's no need to offer points - and moderators don't like it...
You might get that error message if (say) a BDOC is sent to create a case with GUID xyz, but it fails.
A later BDOC, that tries to update this case, will get the message 'GUID xyz does not exist'.
How to avoid this? Monitor the queues daily and do not delete queue entries!
cheers
Paul