Every time I warp to a dimension my server timed out for ages(never saw the end of the timed out... I reboot it after five minutes.). I isolate the error at pixelmon because if i uninstall it the dimension work fine, but once I put it back on the problem come back. I can't give a crash log because the sever doesn't crash... We are just unable to connect back...

Hmm, hard to say what's wrong. Can you test if this is also the case with mystcraft?

Already tested and it's the same =/

I would report this to the pixelmon guys then because it appears to be a problem on their end.