I've got the exact same issue, since I activated the user-profile plugin, I always get this pup-up when I try to register a new person or when I try to change usersettings. If I click OK, I can register a person but not with the additional information, just the basic ones you can without the plugin.
Help would be really great. I could imagine it's a problem of permissions or a problem of the template (read it somewhere).
EDIT: Ok, after searching some more, I found this:
http://code.google.com/p/joomlaworks/is ... ail?id=133
Brought me to the idea, that k2 maybe has something to do with this error(Yes, I am actually using k2)
So what I did: Disabled all the k2 plugins and found out: It is the system-k2 plugin thats messin up my user-profil plugin....have to figure out now why but it's a start...
probably more help here: http://getk2.org/community/New-to-K2-As ... o-fie#4710