We have just released a Hotfix for ArchiCAD-14, you might want to try if that fixes it. (It is not currently tested for ITA language version, though, a few days...)
Generally "Server returns error code" means that the server refuses to accept your sent changes, because that would mess up the database on the server. Typical example: You have modified an element that - according to the BIM Server - is not yours. This is more than likely a program error, but the BIM server tries to protect the central database. In such cases, your best bet is to save a PLN, then reload the project from the server, and copy changes manually from the PLN to the Teamwork project. Alternatively, you can re-share that PLN, if that makes more sense in the current situation.
I tried to do my best explaining these teamwork error scenarios in this article:
http://archicadwiki.com/Teamwork/SendReceiveErrors
Gergely Kmethy
VP, Customer Success, Graphisoft