I am have this same problem posted by someone from 2017.  Exactly.   Also when trying to save it says please fill in highlighted required fields but there are none??

Anyone have a fix to delete this?

Running 15.6.6347
 
One of the admins tried to rename an account.  They copied the display name which contained @domain.com in the username field I assume.... SM renamed that account from user xx to xx@domain.com.  All other usernames are xx format and don't contain @domain.com....  I cannot delete the user in the web interface.  When pulling up the account it gives a different page with fields for domain, active directory user name, etc... basically looks like its every possible field for that page.  When you try to fill it out, it won't save.  You also cannot rename the reconfigured account.