opc and wincc ..

i try to communicate between two wincc 6.0 stations (2003 under vm), but opc does not work.

it works on the servers, as long as they are "standalone", without membership in 2003-domain.
when i make them domain-members (not dc), opc and opc_xml no longer communicate.
installing connectivity pack and liscence did not help.

i need the domain, because the systems are used by students -> many accounts !

any ideas ?