Is that some update has caused a slight issue and when a user logs off the server isn't releasing that Log In Authority. Of course that is assuming that you have set a high enough number of users who can log in.
So assuming that it's just an issue with a Update. Hot Fix not being quite right most times some simple maintenance cures this so run the obvious things that you do with your normal Housekeeping and see if that fixes the current problem. Remember to reboot the system after finishing the cleaning.
Col
but my first thought is maybe a configuration issue as to the maximum number of connections, you may need to check and verify you have alloted enough for the number of users.
Connectivity issue with my server on a particular port