Ok, i have installed the latest Live messenger, I went in to the control panel and pressed preview screensaver and waited a couple of seconds. And when I exited the screensaver I was logged out, as usual. Then, I tried to select another screensaver, and it worked!
Apparently, the screensaver I was using (
pixelbreaker : PolarClock) has issues with the latest versions of messenger... I'll just have to change screensaver
.
Thank you for your help!