HTTP 403.9 – Access Forbidden: Too many users are connected

I was trying to test multiple connections to my local IIS server when I
got the http status/error code above. This is because there is a limit
to the number of concurrent connections with Keep-Alive Enabled setting

So I unchecked "HTTP Keep-Alive enabled" and the issue was resolved.

when I run/debug from Visual Studio (for an application configured to
run on IIS) i got an "Unable to start debugging on the web server. An
authentication error ocurred while communicating with the web
server…". IMHO, the error messages doesn't quite help in debugging
but it turns out that Visual Studio just needs the the keep-alive
setting ON to work for applications configured to run on IIS. Turned it
back on and worked fine.