Today I noticed the same problem, I was trying to edit a user (I was changing the applications they had access to), I’d click save and nothing would happen! There was no error, there was no message from Umbraco.
It was as if the Save button was disabled.
And then it hit me, I was using Firefox and I had saved the username/ password combination when I logged into Umbraco.
That makes sense, the error seemed like some kind of client validation had been failing, but there was nothing obvious to me.
Then I realised that the password fields for the user details were hidden, so I clicked the Change Password link and look there, I have an error saying that my passwords don’t match.
Because I’d saved the username/ password in Firefox it was kind enough to auto-complete one of the password fields (using what ever algorithm Firefox does to detect password fields), but then there was nothing in the confirm field, causing the compare validator to fail.
So I cleared the field and clicked save, and all is well.
But why did it work when we changed the username? Well then Firefox detected that there was no password for that user, so it cleared the field, and the compare validator passed (it don’t compare empty fields).
Lesson learnt – be careful when you’re storing passwords :P