I got the message when clicking 'Register' at the bottom of the registration screen. Unfortunately, I am unable to reproduce the error on a fresh install of WP and Cimy User Extra fields.
I had disabled all plugins other than UEF and switched to the default Twenty Twelve theme to try to narrow down the cause, but still got the error. Here is the environment:
Several required fields for registration
I did a grep for 'cimy_update_ExtraFields' in the plugin files and found 3 references:
- the function definition at line 522 of cimy_uef_profile.php
- a commented out add_action at line 563 of cimy_user_extra_fields.php
- an add_action('profile_update'...) at line 566 of c-u-e-f.php
I made a guess that cimy_uef_profile.php was not getting loaded as I could not find a require for it. So, I added the require and the problem disappeared.
Perhaps some other plugin did not clean up after itself when disabled?