I think this is because of the new ‘required’ feature that been implemented recently. Do you have a required custom field at signup that was introduced at a later date and is not editable after signup? (ie do these existing users have an empty required custom field that they can’t edit?)
If so, you can temporarily change the custom field to allow edits or make it no longer a required field. This should allow people to continue using your site while a fix is being worked on.
Checking in on the progress, I believe a fix is already in place
Could you update your site and see if that fixes things?
