We've introduced a patch a while ago in order to deal with a problem of Microsoft IIS, as explained on
That solution was working fine until we upgrade the HttpClient code required for the SeleniumMonitor, and now the Host header always contains the port, for example:
The protocol specifies that the port is optional and when it is not specified, the default port for the chosen schema will be used. But, it is perfectly valid to specify the port even if it is the default port for the selected schema.
IIS returns a 503 error if the default port is specified, which is clearly a problem of this particular web server.