Steps to reproduce:

1. Log in to back office.
2. Go to Admin / Users / Shop users / Private customers.
3. Add new user with filled in fields Salutation and Customer group.
4. Edit created user.

Actual result:

See attached screenshots.

Newly created users with filled in fields Salutation and Customer group aren't editable because an error 500 occurred.

Expected result:

User can be edited and no exception occurs.

Note:

An error 500 occurs also for creating new users with filled in Customer group field.