>>We had such problems before in my former job place. Once the file got locked, only re-booting the computer in question released this lock.
>
>We had to reboot a few times, too. At other times, I just boot the users (if a file called "maintenance.tmp" exists, the application quits automatically), but this doesn't always work.
>
>And, sometimes the problem disappears by itself after a few minutes.
>
>The problem is, this is happening almost every day, recently.

Sounds like some weird Network problems. Do you have a Network Admin whom you can work together on this problem?