If a member goes to connect their Facebook account while they are "Acting as a business" they will be prompted to switch. This "Switch" event will cause Memberstack to error out.
To fix this issue, the member must switch accounts, and return to the signup, login, or profile form to try again. It will work the 2nd time.
Image credit: https://www.guidingtech.com/what-is-switch-account-facebook-and-messenger-how-to-use/