Azure app service some requests returns 400 Bad Request. The request could not be understood by the server due to malformed syntax - azure-web-app-service

Question

So we have a simple .net core 5.0 service that only serves some simple pages with mvc. We are starting to get 400 Errors (details below) on some of the requests. Our frontend is embedded in an iframe which forces us to use our own domain for our api-calls. The 400 errors disappears when we use the azure internal-urls. (*.azurewebsites.net instead of *.ourdomain.net). When I get to the "diagnose and solve problems" -> "availability and performance" -> HTTP 4XX ERRORS i can se below errors. Any ideas on what can cause this error?
Bad Request. The request could not be understood by the server due to malformed syntax. The client should not repeat the request without modifications.

Answer 1

So, the biggest problem above is that we do not get the correct errormessage. After a lot of experimentation we activated the ConnectionLogging for Kestrel.
WebHost.CreateDefaultBuilder(args)
.UseKestrel(options =>
{
options.ConfigureEndpointDefaults(listenOptions =>
{
listenOptions.UseConnectionLogging();
});
})
And after that we found some more intressting logs. One that said:
Connection id "0HMFSA73IA4LS" bad request data: "Malformed request: invalid headers."
Microsoft.AspNetCore.Server.Kestrel.Core.BadHttpRequestException: Malformed request: invalid headers.
After some more investigation we could diff a succesful request from a failing request. And the problem was related to the certificate of *.ourdomain.se. In a part of the certificate we hade a string thats named "Stockholms län" in the cases where it failed the string decoded to l�n and when it succeeded the string decoded to l%C3%A4n. We are now investigating if this is a load balancer problem. But this app is running hostingmodel outofprocess. By changing this to inprocess and wrap our Kestrel in IIS the errors disapears.