Hi there.
Yes it is an issue in 2.10 release, fortunatly the fix is super simple while you wait for the fix to get into a release:
Open the file: bb-plugin/classes/services/class-fl-builder-service-convertkit.php and change line 204 to this
public function subscribe( $settings, $email, $name, $custom_fields = array() ) {
We are declaring the 4th arg optional which will fix the fatal error.