DatabaseError when changing username as admin

edit

I get an error (I get an email from my site) when I'm trying to change some users name:

DatabaseError: record "new" is not assigned yet
DETAIL:  The tuple structure of a not-yet-assigned record is indeterminate.
CONTEXT:  PL/pgSQL function "group_membership_tsv_update_handler" line 6 at assignment

the commands

python manage.py migrate
python manage.py collectstatic

don't solve the problem.

any ideas?