That’s a message from the Relay that it is too busy to handle the client request (usually a report posting, but I suppose it could happen for a download too?)
The Relay may be getting the same message from its own parent, all the way to the root server in some cases, so check the connectivity and performance all the way up the Relay chain.
Often you might find one that is stuck with reports in it’s FillDBData\BufferDir directory. You would normally see files created here, then almost immediately removed as they are sent up the next level in the Relay chain.
