Hey again,
I haven't been able to replicate your problem, but let me ask a couple of more questions:
1. Are you trying to add your category through this add interface?
2. What browser are you using (Internet Explorer 8/Safari/Chrome/Firefox)?
Let me know the answers to those questions and I'll keep investigating.
I'd also recommend trying to disable other plugins one-by-one to see if there is some conflict happening.
Thanks!