Error adding user in newsletter form in Zoho Commerce

Error adding user in newsletter form in Zoho Commerce

1. Our goal: We let customers subscribe to the newsletter of the Sol shop.
I've connected this instance of Zoho Commerce to this contact list in Zoho Campaigns.
When one tries to submit data in Chrome / Edge using the form at the lower end of the page, Zoho responds: "Error in adding user"
Let's find why and make this form work!

2.1 Maybe you've discussed this here, but it doesn't seem easy to find information about solving this error.
When you improve the search in Zoho Desk, one can find more information, so you reduce customers' costs. We can help.
2.2 Another issue of Zoho Desk: When I edit such a message, it unsubscribes me from this conversation.
I loathe such reactions of software.
Let's change this now! Let's talk this week!