Related to JSP-73729

I am using the "External System Import - Beta!" option from the Import & Export section of the left hand menu under Administration. I then choose "Mantis from 1.1.8 to 1.2.4". I have already created the configuration file so I expect to just fill out the fields and move to the next stage. However I keep getting an error "Mantis not detected at given URL", although I'm sure the URL is correct (http://mantis.corp.mycompany.com). Mantis is hosted on IIS, there are several other websites on this instance so I can't use http://localhost. Which page should the URL go to? Just
the Mantis homepage? And what could be possible reasons for Jira not recognising it being there?

To use Mantis (in Internet Explorer) you go to http://mantis.corp.mycompany.com and this takes you to the login page.

IIS, the Mantis database and the Mantis website all live on the same server (Server1), there is a DNS alias (CNAME) record that points mantis.corp.mycompany.com to server1.corp.mycompany.com. Within IIS, mantis.corp.mycompany.com is bound to the Mantis website on port 80. This is the only host name bound to the Mantis website. There are other websites hosted on IIS on Server1, but they are bound to other host names.