Jon, really difficult to tell with what little detail you've provided.
Are you getting any errors at ULib/ULX startup?
What are you typing to ban?
Are you getting errors when you type the ban command?
Can other admins, if you have any, ban?
Are you using our current release, 3.2? Or are you using SVN? If SVN, what revision?
Seige is correct about current release. 3.2 requires ban time and reason (though, minutes is assumed Seige. The way you are banning would make 'minutes' the reason for ban, not anything to do with actual time)
This is a bug we've fixed for next release.
Did you try new addons?
Some addons monitor chat text.
Think back to the time it started happening... what changed on the server?
If other admins can ban, it might be a clientside scripts issue on your side.