Every now and then, the following things stop working on my server (all at the same time) until i reboot it:

ftp passwords, ssh password & web login to directadmin control panel, website session ids (i use these instead of cookies).

apache still works fine, my websites show, but you cant click through the pages on my sites as the session id doesnt work.

It seems to happen at periods of around 2 months, but im not sure if its exactly 2 months or just around that time period. It doesnt seem to happen straight after a reboot or close to one anyway, always after a few weeks.

Heres what I have checked so far:
diskspace - loads left
/var/log/messages - nothing suspicious here
/var/log/directadmin/error.log - nothing suspicious here

in /var/log/directadmin/security.log I see line after line of *** <ip address> has tried to login with an invalid username: '(null)' ***

could this be anything to do with it?

any advice, things to check, possible causes etc.. would be greatly appreciated!