It seems a 3rd party user field may have an error causing the issue.
Go to PHP MY ADMIN and look for fusionxxxxx_user_fields, click Browse, look through your fields and drop the ones that are not included with the PHPFusion Core package one by one until you find the bad one.
Hope you fix it and find out which user field is the culprit.
I took your idea a little further. I did a "truncate" on my user_fields table and added every field via the db, as it seems you can't add the field via PHPFusion when the field is already in the user table.
After every new field I delete and re-added the core yahoo user field, just to be sure it all still works. Now I have nearly the same situation as before: I can delete and re-add the core user field yahoo without any problems; But I still get the blank when I add the core (!) user field icq. I already downloaded the PHPFusion source and overwrote all my userfield php with the source... No effect.
If I could see an error message!
Merged on Mar 30 2014 at 00:27:13: OMG I gone nearly insane; The problem was my theme BSF. I'm not really sure why, but I can manage my user fields now without any problems.