The site has a log located here:
http://dicomserver.co.uk/logs/ That was where it told me the the fields were null and was refused. This site seems to be very picky it forces you to be compliant to the the dicom spec. I was able to get the worklist working but never got the site to take a file. I have also been using Orthanc as the pacs which does take the files and visualizes them for you in a web browser. That is how I found the actual fields that were null. The site is located here:
http://www.orthanc-server.com/ This software doesn't have the worklist implemented yet so I use the worklist settings from the uk site and the pacs to the local orthanc server.
As for the logs at the uk site they are arranged by date and time. Since it is made for developers it gets hit by many companies testing their software. They recommend, setting the client AEtitle to something unique so you can search for it in the log file. Also if the log gets filled fast enough the site will generate multiple log files and each test appends the log so your one test can span 2 different logs sometimes.
I didn't know there was a log inside Medmont. For the computer I was testing on. That computer got re-imaged and is currently being shipped to a customer. So I no longer have the computer. If you search the logs for the uk site. I'm sure your being denied for the same reason so I would check the log for the test you already tried on the site. I hope this helps with your companies testing and I was able to help you go in the right direction.