XF 2.1 Can't change user name. System reports the name is in use even though it isn't

[Stuart Wright]

I'm trying to change the name of a user. The new name was in use by a member 10 years ago, but that's been changed to something else.
(I double checked by looking via phpMyAdmin and the name is not in use).
The system won't let me change the name:

Oops! We ran into some problems.

• Please enter another name. The specified user name was previously used by another user.
• Please enter a valid name.

Allow re-use of old user names is enabled

Any ideas?

 

[Paul B]

Allow re-use of old user names is enabled

Where are you seeing that?

I don't recognise it as a core option.

 

[Stuart Wright]

It’s not. Thought it was, but it’s a bug in user essentials addon. Thanks for investigating and apologies.