Changing username fails

I am trying to change my username, for example from aaaaaa to bbbbbb. This fails with an error 500.

Here's what I see in /var/log/gitlab/gitlab-rails/application.log:

Started PUT "/profile/update_username" for 127.0.0.1 at 2015-01-16 17:34:29 +0000
Processing by ProfilesController#update_username as JS
  Parameters: {"utf8"=>"✓", "user"=>{"username"=>"bbbbbb"}, "commit"=>"Save username"}
Completed 500 Internal Server Error in 26ms

Exception (namespace directory cannot be moved):
  app/models/namespace.rb:82:in `move_dir'
  app/models/user.rb:517:in `ensure_namespace_correct'
  app/controllers/profiles_controller.rb:44:in `update_username'

I am running GL Omnibus 7.6.2 on Ubuntu 14.04 64bit. Note that I am at then and of a long (but issueless) migration path:

1. initial GL 6.3 on Ubuntu 12.04 using MySQL
2. upgraded to 7.6.2 src on same machine
3. migrated to PostgreSQL
4. migrated to new machine using 14.04 and 7.6.2 omnibus