Works, unfortunately only on the monitor, not on Beamer

I bought the adapter for lectures at the university.

At home I immediately checked him on the function. The monitor everything worked great.
The next day in the Unihabe I tried to connect the adapter to any of the projector, no chance. The adjusted resolution, search for monitors, no success.
After the disappointment at the university, I have at home again carried out a test on the monitor. The monitor everything worked again.

The next day at the university a Professor had his MacBook with an original mini Displayport adapter (Apple) to lecture there. I have used this situation as a sample for example and connected my first adapter to the projector, no function. Immediately thereafter, I used the Apple adapter of Prof. and lo and behold, no problems.