<TL;DR;>This error message is not always caused by proxy issues. After last weeks updates an old version of the service bus DLL’s (2.2.3) on the relay server side caused this error on the client side when trying to call service operations.</TL;DR;>

Last week I arrived at the office and was greeted by a status screen that contained a lot more red lights than when I had left the day before. That in itself wasn’t too strange, we monitor customer’s servers as well and who know what kind of update/reboot schedule these guys have. However, the fact that the only servers that were experiencing problems were the ones we host ourselves made me a bit suspicious.

After some investigation I noticed the error message from the title in our logging. Apparently it can be found in two variations: 50200: Bad Gateway, and of course 502: Bad Gateway. I had encountered this issue before at a customer using a proxy, and all google pages led me to believe that this was indeed a proxy issue on our side as well. However, we don’t have a proxy running in our network, and it was working fine before.

After some digging I noticed only the servers that received updates and were rebooted the night before were experiencing issues. Servers that had not been updated were fine. It turned out that one of the updates did not play well with the old (2.2.3) version of the service bus DLL’s we were still using (software had been running fine for 3 years, why update?). So after updating it to the latest version that could still run on .NET 4 (2.8.0 if I remember correctly) and updating the software on the rebooted servers, we were back in business again.