Hi,
after some tests i have found out the following:
When we connect through a UMTS-Device the proxy isn't working. We can reach the intranet but the traffic from the browser doesn't reach the proxy-server.
When we connect via LAN or Wi-fi the proxy works and we can reach web-sites.
I have attached a screenshot where you can see that the setup doesn't install an element at the settings of the UMTS-Device (Aruba Via Driver). Can this missing element cause our troubles?