Stripe Customer (find): contact_id is required
I'm getting this error using CiviCRM 5.3 while registering for a paid event as an anonymous user and registering with contact information that is not already in the database.
In around line 600 of CRM/Event/Form/Registration/Confirm.php, there is:
list($result, $value) = $this->processPayment($payment, $value);
The $value array doesn't have contact_id in it (even though $contactID is available to it).
I'm not sure if that is different in more recent versions of CiviCRM.