Just to follow up on this: in a previous version we had an issue where a page required a field to be filled out, but if it wasn't, you couldn't make any changes to the page. That is, the Save button wouldn't activate if you DID change something. However, there wasn't an error thrown on the field when you opened the page...
To see if this is the case, use your TAB button and open the user, then tab through the various fields on the page. When you pass a required filed that isn't filled out, or filled out improperly, you'll see an error display. Fix that issue and you should be good to go.
Now, I never saw this when editing a User, but I did see it when editing a domain or editing a mailing list...so my hope is that the same applies here.