Strange behaviour of Only One account

[amleto]

YaBB SE Version: 1.5.4
PHP Version:
MySQL Version:
Server Platform: Unix, Linux, or BSD
Link to Forum: http://utenti.lycos.it/astronomican/forum/index.php

Problem Description:
HI, a freind of mine has reported to me a strange thing that has happened to him yesterday using the forum. He received a message something like "someone maybe have tried to enter your PC" or something similar...not from the forum error message I think, though.

Coincidence or not, after that he could not do anything with his account. He wasn't able nor to log out from the forum. Eventually he succeded to log out and log in again but after that he coud not post again.
Pratically he could only log in and read the forum as every time he posted the forum took him on the index and didn't displayed the post.

How can this be?

Thanks

[[Unknown]]

Maybe his comp[uter was hacked... I don't know.

-[Unknown]