Today at 12:00 I changed the DNS Entry to point to an Application Gateway instead of Azure Frontdoor. After doing that there were some failures with status code 0. The funny thing is that only POST requests were blocked. GET and Option Requests sitll worked fine. Also in the log the Application Gateway didn’t seem to resolve the route to the server. The question is whether this behavior is a client side firewall issue or actually a problem with the App Gateway.