Migration Suite for SharePoint and Administrator for Office 365

Unable to establish connection to SharePoint using trial of Migration Suite for SharePoint

I’ve downloaded the trial of the software and installed it on my local machine. I've tried to connect to SharePoint using the button on the ribbon but even with farm level credentials I can't get it to connect. I've tried pasting the url direct (without the sitepages.home.aspx on the end) and I've tried finding it through the "Discover Site Collections" wizard but nether option gets me anywhere. With the former I can input the details and click OK then I get an error message "Connection Failed: com.sun.xml.internal.ws.client.ClientTransportExcepton: The server sent HTTP status code 503: DNS Fail".  With the latter I can get as far as seeing the sites listed and select the one I want but then the Next button doesn't do anything to allow me to continue with the wizard and the OK button puts me back in the same place with the same error as I just mentioned.

When I activated the product it worked without needing the proxy configuring and the SP environment is hosted on premises so shouldn't need the proxy anyway. I know the credentials are ok as they are valid for farm admin. We're using SharePoint 2013 Standard edition, hosted on premises, and I have made sure I'm on the latest version of the Migration Suite.