Fatal error in membership 2 Pro when adding a user synced via User Sync

On a single site where users have been added via the User Sync plugin. When trying to add any of those users to a membership in Membership 2 Pro consistently returns a blank page with the following error:

"Fatal error: Uncaught Exception: You cannot register a new account, because you are already logged in. in /home/***/public_html/**.usrec.com/wp-content/plugins/membership/app/model/class-ms-model-member.php:990 Stack trace: #0 /home/***/public_html/**.usrec.com/wp-content/plugins/membership/app/model/class-ms-model-member.php(939): MS_Model_Member->create_new_user()
#1 /home/***/public_html/**.usrec.com/wp-content/plugins/membership/app/controller/class-ms-controller-member.php(441): MS_Model_Member->save()
#2 /home/***/public_html/**.usrec.com/wp-includes/class-wp-hook.php(286): MS_Controller_Member->members_admin_page_process_editor('')
#3 /home/***/public_html/**.usrec.com/wp-includes/class-wp-hook.php(310): WP_Hook->apply_filters('', Array)
#4 /home/***/public_html/**.usrec.com/wp-includes/plugin.php(453): WP_Hook->do_action(Array)
#5 /home/***/public_html/**.usrec.com/wp-admin/admin.php(214): do_action('load-membership...')
#6 {main} throw in /home/***/public_html/**.usrec.com/wp-content/plugins/membership/app/model/class-ms-model-member.php on line 990"

Manually adding a new user to the site and assigning a membership works just fine though.