I am also having this error on my website as well – I would like an answer from Ultimate Member if possible too.
The issue seems to be with the default registration form (but not sure if it affects custom forms) and the options for accounts. My issue may be more complicated because I have TWO user account types as “Customer” because the Ultimate Member plugin added a “um_customer” to WordPress despite the fact that I already had a “um_customer” account type already available. So I have TWO “customer” account types and I feel that may be adding to the problem as well.
The difficulty with that is that I cannot delete either account type because when I do, it ALWAYS removes the one I did not create instead of the one I did no matter which one I delete, and that screws up every single account (all 700+) on my website with that account type assigned.
HELP!