I'm having almost the same problem, but with a little bit differences, like:
I use 10.6.3 in a C4, now it upgrade to C5 (L2Cr -

), and when I try to login with my old walker it says: Login LoginServer Fail.
I have already change the IP, port, Token... And still cant get it to work. I got the version 10.7.2 from a friend, but i could only open it once. After i close it and try again it says: "l2walker.exe found a problem and need to be close"
So, now i can't login with 10.6.3 (And the guy above me can. lol) and neither use a new version... Any tips? Plz... I really need it. =X
Thx in advance.
EDIT: GriFFit3r, i downloaded ur 10.7.4 ultimate version, but i keep getting that l2walker.exe error, and I cant even open the walker. ><
Any sugestions? Plz =X