From the information provided, it looks like it is exactly what it says:
"Mail folder is missing on the server".
I suspect that it might be a folder that exists on your phone and it doesn't exist on the server, and that folder is configured to be synced.
You can check in the Account Settings -> Folders which folders are configured to be synced, and that might give you a clue.
Alternatively, you can create a log, and catch that error in the log, and see yourself which folder is giving the problem.
Kostya is on vacation now.
You can create a log and e-mail to him (check how to do that in the signature of any of his postings), and he'll respond to you with the details on or after September 18.