Change in AD Authentication source and the UI shows no Users in Settings > Edit User

We have changed AD’s for authentication providers and while this seems to have worked seamlessly - i.e. I have been able to log in and authenticate against the new domain - when I now go into Settings > Edit User > the drop down user list is empty.
This is causing grief with the Oxidized API key as I need to re-issue this due to the AD change and I cannot select a user to put it against.

Any ideas how I can resolve this and get a list of users back in the UI?

Thanks!