Domino.Doc 3.0 won't have been tested with Domino 5.0.12 as 5.0.12 was released some time after Domino.Doc 3.0 and we don't backtest previous versions.
The latest version of Domino.Doc, 3.5, is supported on 5.0.12
http://www-1.ibm.com/support/docview.wss?rs=203&q=7002980&uid=swg27002980&loc=en_US&cs=utf-8&lang=en+en