digiteye, that's a software bug. We had the same issues on a german forum. In case you are towing a big trailer and the X has to work hard and is getting hot, the car will switch to a emergency program deactivating every non essential device (including air condition). The bug is that the car switch to that emergency program every time you connect something to the tow bar connector.

Try this. Start the engine and connect the trailer instead of connecting the trailer and start the engine. Aircon should work if trailer is connected while the engine is running . The colleagues here on the German forum got a new software, BMW is aware of this bug, so tell your dealer that there is a update which fix the issue.

bye
Sven