Strange asterisk registry problem

I do have a very strange problem i never had before. I do got some asterisk-servers, in two different locations. To one location (SL1) i can’t register sip (registry timeout) from office, but from home. To the other (SL2) location i can connect both from office and from home. Exactly same settings, tried from different computers at office, but same problem. I do have 2 different internet providers at office, tried both, same result. What can be my problem?

I do got same settings on all servers and if it’s a nat-problem, why can’t i connect to SL1 from office but from home, when i can connect to SL2 from office and home? The settings in routers are the same, both on SL1 and SL2. Asterisk-setup is the same, same versions (asterisk 1.2.17, zaptel 1.2.16 and libpri 1.2.4) and same operation system (ubuntu 6.10 lamp-server). I know it’s not the latest versions to, but i don’t think it’s a version-related-problem.