domain trust validates OK, but not really.

hello.
i have a domain X on 2003 servers (2 of them)
i have a domain Y on 2008 server & 2012 server.

i successively created a two way non transitive trust between them, originating the trust request from domain X servers.

• i can validate both from both domains.
• i can share folders on domain Y for domain X users.

but

• when i try to share folders on domain X for domain Y users

i get: "the following error prevented the display of any items the server is not operational" in the Select Users dialog of the folder permissions option.

• I created a user of same login and pw and shared it in the dns of both
• i created manual forwarders (host A) in both domains to both servers
• i can ping the server names from all servers/domain and resolve the IPs
• i also added all server entries to both domains fw and rev looking zones (not sure if its necessary, or if it actually causing issues)

what am i missing?