This is SOP for any membership site built with WishList member plugin
1. Login as admin
2. Go to Users
3. Paste the email address of a user who has problems into search field
4. Click edit user
5. Make sure Nickname and the email address are DIFFERENT
6. Click Generate Password.
7. Make it 8-10 chars long using caps, numbers and symbols. Make it aesthetically nice looking:)
More in the video: