Just for clarification, it looks like you're using the SAML product for ASP.NET rather than ASP.NET Core.
Do you get the same error if you use the ExampleIdentityProvider and ExampleServiceProvider projects that we ship?
This will help identify whether the issue is associated with your environment or application.
We test with all the major browsers including Chrome and haven't seen any issues.
I suspect that hitting the keyboard causes an HTTP Get to your endpoint which results in "The message is not an HTTP POST" error.
I suggest user Chrome's browser developer tools to take a look at the network traffic.
You should see an HTTP Post with SAMLResponse post data being received at your assertion consumer service endpoint.
If there's still an issue, please save the network traffic to a HAR file and send this to [email protected]
Also enable SAML trace and include the generated log file as an email attachment.https://www.componentspace.com/Forums/17/Enabing-SAML-Trace