After some issues with my apache configuration (cp. QGIS-Server - 500 internal server error) and some playing around with setting up things correctly like projections, service versions,... I succeeded in setting up a QGIS Server WMS wich works great (transparency, querying, ...) ...in QGIS, as can been seen in screeshot below (the yellow parts):
Unfortunately, in my organisation some people are using gisses different from QGIS. Problem arises e.g., when trying to display the same service in ArcGIS 10.4 (via ArcCatalog, Add WMS Server), set up like this:
My ArcGIS Project is set up the same as my qgis project (crs=EPS:31468), the service seems to be drawn (note the status line: 'Drawing...') but nothing is shown at all, except the basemap:
Furthermore I tried to add the WMS to an ArcGIS Online map, with no success at all...
...and the following error:
Since I use an internal URL, Firefox is configured to connect to the internet without a proxy. Directly typed into the browser, the GetCapabilities request works: