ApplePay Session 400: Bad Request

Suddenly I keep getting the following error:

"HTTP Status 400 \u2013 Bad Requestbody {font-family:Tahoma,Arial,sans-serif;} h1, h2, h3, b {color:white;background-color:#525D76;} h1 {font-size:22px;} h2 {font-size:16px;} h3 {font-size:14px;} p {font-size:12px;} a {color:black;} .line {height:1px;background-color:#525D76;border:none;}HTTP Status 400 \u2013 Bad Request"

Apple Merchant Identifier certificate and Apple Payment Processing certificate are fine. Everything was working fine some days ago.

The domain has been successfully registered as well.