The content of your message is causing problems. Open your campaign and verify whether or not the message respects the LinkedIn character limits.
If you're using one of Stent's predetermined values (your contact's first name, for example), make sure to leave some space in the message, since your contact's name will be counted in characters.
Your invitation message contains 294 characters out of the 300 allowed. If the name of your contact is Carl
, the message will be sent. If the contact is named Christina
, an Error Payload will be triggered.