Error on registration

Just Upgraded from 3.7.1 to 4.0.1 on Joomla 3.9.2 site. Tried to register new user with the setting to "Block Non-Subscribers" and force them to pick a subscription. After I registered, chose the default 'Forum Access' and checked out, I received the following error:

0 Serialization of 'SimpleXMLElement' is not allowed