Azure B2C API Connector fails with http: 400 There was a problem processing your request. Please try again

Question

I've connection issues between Azure B2C API Connector and a CAPTCHA Web-Service. Everything worked well until 31.12.2021. Not sure about the first week in January 2022.

All services, whenever offered by Microsoft are in a Swiss data center. Since January I'm playing Mail Ping-Pong with the MS Support Team :-(

• Azure B2C calling a service via Azure Front Door in detection mode fails.
• Azure B2C calling a the service "direct" works.
• Calling the service using Postman works via the Azure Front Door and Direct

I don't use custom policies, so no logs. Or maybe I don't know where to look? Azure unfortunately only returns a HTTP 400 with the message "there was a problem". The Service is not receiving any get request. According to the Firewall log, no request is blocked.

Would be happy for any hints. My personal best guess - Microsoft did renew the X.509 SSL certificates and forgot one due to the 31.12.2021

But would be happy for anything else that might had changed?

Thanks a lot.