Another Notification Settings with name xxx exists, select another name

I have installed ERPNext on our own servers, and have been customizing the same using a mix of imports and actual customization on the app itself. However, I have a problem: one of my users is unable to work on the system. I have created and assigned them a username and assigned access rights as I did with everyone else. However, she can’t see staff expenses for example, even though she is an expense approver and the workflow I created has her appointed as such.

I realize I had earlier created a different username for her using a different email, which I then disabled. That username was able to access these expenses. The new one, with similar rights and roles, doesn’t. I tried to merge the two and received the above error.

What might I be missing? How could I fix this?