after a long long long test in my office... I maybe have to tell you this... I haven't this problem before installing WLM 9 in this PC, then I decided to uninstall it and downgrade to WLM 8.5... and the problem appeared.. I am not so sure if the problem began in that moment or days after... so I can tell you if the issue is because the uninstalling of WLM 9 and reinstalling of WLM 8.5... but this can be the real reason...
I decided today to install WLM 9 again to do a test... and the test failed... cause with WLM 9 (with MP!L enabled or disabled) my first message to a contact was sent but all the others after that one got lost... so I downgraded again to WLM 8.5 and the problem (without using a skin) began to appear... so I decide to restart messenger disabling MP!L and something very weird happened... all worked right
...
parallel to this I turned on my laptop (the one is conected wireless to the same proxy server) and the WLM worked perfect with MP!L, and for your surprise SKINNED
...
so I don't know what the f*** is happening here but is very very very weird... and seems to be a problem with MP!L 4.5 and my office PC (maybe for the installation of WLM 9)...