Not able to delete Users from the sandbox environment

I'm currently trying to delete users I'm using for testing purposes in order to be able to run more tests on my Sandbox environment.
I have disabled the desired user first, as is required to have the Delete option be available, but am not able to delete it.

- I have tried using the tickbox next to the User's name and clicking Delete in the top ribbon. I get the pop-up warning about deleting users and how it cannot be undone. I proceed and see "Bulk Operation in Progress" going, and then finish without any further notices or errors, but the user is not deleted.

- I have also tried using the three dots on the right-hand side of the user and clicking Delete. Similarly, I have the "Are you sure you want to delete the selected user" warning message. When I click on Delete, I get an error message with "We are sorry. Your request cannot be completed at this time. Please try again later.".

I initially thought it might have been a temporary issue, as was mentioned in another Question on these forum last year, but it's been three days and I still cannot delete that user.
I have tried from 2 different browsers (Chrome and Firefox) just in case, and the results were the same.

My user is Super Admin on the Sandbox environment, so I don't believe it would be a permission issue either.

What else can I try? Thanks!