Zoho Flow to Campaigns - "Error in subscription. Please retry after sometime"

I am running a flow using the following apps:

Bookings

CRM

Campaigns

The Flow is triggered whenever a new Booking is scheduled. I am using the bookings appointment details to:

filter through a decision tree with different outcomes,

return the CRM contact details,

Add the contact to a Campaigns List

When testing it always fails at the last step of adding the contact to a Campaign List with the output error Zoho Campaigns says "Error in subscription. Please retry after sometime."

The input data is correctly formatted and appears to be correct:

{
	"Address": "The Hive, 5 Tai Mong Tsai Road",
	"Spiritual": "",
	"First Name": "Tim",
	"Website": "",
	"Job Title": "",
	"Skin Wellness Consultation": "",
	"Contact Email": "tim@xyz.com",
	"Twitter Handle": "",
	"Zip Code": 999077,
	"Phone": "+85212345678",
	"Portal": #####2168, (Obsfucated for posting here)
	"List": "8298807e4bd6c7486705efb07187d539c98e811#########", (Also Obsfucated)
	"Secondary Email": "",
	"Linkedin Handle": "",
	"Lead Source": "",
	"TEST FIELD": "02-Jul-2020 21:45:00",
	"Facebook Handle": "",
	"Title": "",
	"City": "Sai Kung",
	"Mobile": "",
	"Company Name": "",
	"State": "New Territories",
	"Dermapen Consultation": "",
	"Note": "",
	"Skin Consultation": "",
	"Country": "Hong Kong",
	"Massage": "",
	"Last Name": "Sparrow"
}

I have re-checked and reset up the connector but it still fails. I cannot find any information on the error other than it is for error code 2001 in campaigns, which doesn't help with debugging.

Any thoughts on what is causing the error and how to fix it?

Many thanks,

Tim