Any idea why this is happening?

[kaamaru]

Read this topic: http://ihackmyi.com/forum/index.php/topic,43039.0.html

Some users are getting logged out and some are not. I've already cleared cache, emptied the sessions table in the database as well as fixed any errors.

[Arantor]

Yes, there's a spate of bots roaming round SMF forums trying to log in as different members, and before 2.0 RC5 this will log those users out. (As of 2.0 RC5, or RC4+security patch, this won't happen.)

[roonekoos]

The security patch don't help on RC4, I had installed it but no good for this issue.

Download the attachement I got earlier today from this forum , it has a list of 1300 of the spam bots.

Copy it into your .htacces file (backup first) and your problems will be gone.

Let me know if it works out for you

Cheers!