I am currently creating a WMTS server and I have tried to connect to it through ArcGis online. I also tried to create a service that could provide single tiles by supplying urls directly and this worked (The tile layer option). My problem occurs when I connect as "A WMTS OGC Web Service". In this case I cannot get any contact to my server. It seems that ArcGis uses a proxy. By monitoring the traffic I could see the following:

The problem seems to be the "/sharing/proxy?" part that is added by ArcGis. Do you know a work-around for this issue or how to set up the server correctly so it becomes possible to communicate with ArcGis.
Any help is much appreciated.