Exchange 5.5 synchronization problem

I have a problem with synchronization of mail items between my Exchange server and offline folders. The error message is:

Error synchronizing folder
[80004005-501-50A-550]
The client operation failed.
Microsoft Exchange Server Information Store

I get the same message when runing autoarchive operation.
Some messages do get synched/archived. I have noticed, however, that problem lies in larger files (more than 50 kB). Those files seem to cause this error, but I have no idea, how to resolve it. Server is configured properly. If anyone can help, I'd be gratefull.