Please help me!

Ive purchased the PRO Version (2.5.4) and everything went fine but when i trie to connect its just working for about 2 seconds and the disconnects emediately! Im going through a firewall with a T1 connection in my work! The System administrators opened the pot 6346 for me, and they dont see any problem!
On my Mac OS 9.2 it works perfectly but with Mac OS 10.1.5 it doesnt!

What can i do?!