The full error message is:
Microsoft.IdentityServer.Web.InvalidRequestException: MSIS7042: The
same client browser session has made '6' requests in the last '7'
seconds. Contact your administrator for details.
There are many causes for this; one being the "missing /" on the identifier.
I found one recently where I was running an ASP.NET MVC application inside VS that was authenticated via ADFS. This used the OWIN WS-Fed middleware.
I couldn't authenticate because of this error.
ADFS will only accept https connections so the RP was configured with a:
But on VS, inside "Properties / Web", I noticed that the URL was:
Setting this to https fixed the problem.
I found a similar solution here.