There are indeed a few companies that won't allow you in their lounges as long as you wear your full uniform. Get the stripes off your shoulders and hide the stripes of your blazer, all fine then. In Europe/Germany they are quite strict about it at Lufthansa. They obviously had some complaints from self-entitled passengers that there were "Lufthansa crew leeching the lounge", although in fact it were just travelling crew members of other companies with a certain status and/or business/first class ticket. As a German, I find the reasoning rather weak, but it is what it is.