[Mrki_Drakula]

What kind of system you are using? Data sistem (.txt,.ini, whatever which is stored in your scriptfiles folder) or something like MySQL system? The reason could be overflow of your database, so everytime someone connects server tries to find the user file, try deleting the inactive accounts if your database is full.

If that is not the problem, try giving out a bit more of informations about your server, your account system, do you have any warnings, errors, or whatever. Use the crashdetect plugin a run a search.