Dell U2414H crashes permanently with several games.


I own Dell U2414H, run with Lenovo x230 and usually plugged into a Lenovo Docking-Station with DP. System is Windows 10, 64.
When I start Games, mainly Bioshock Infinity and Civilization V, it happens more and more often, that after starting the game the monitor flickers. I hear the sound notice for plugging a device in and out several times until I force shutdown the monitor. The Display of my Laptop then works the game without any problem. If I shut the game and plug the Dell U2414 back in, it keeps the bug: black screen, sometimes jumping back to the Laptop-screen, plugin-plugout noise.
If I restart the computer, the monitor works just fine, until I start Bioshock ininite or Civ V. The setting with the same monitor has played BIoshock infinite for weeks without the problem. After yesterday it keeps starting only with the bug. With Civ V it has the problem on every third gamestart or so.
No changes in the system can be related to the bug.

I tried to switch to USB- connection and plug in directly at the computer - same problem.
I resetted all user-personalizations - no effect.
I can't read out my Service-Tag number, btw., even after installing all the tiny little programs Dell wants me to install: 
"Die beim Scan ermittelte Service-Tag-Nummer konnte nicht erkannt werden. Führen Sie den Scan erneut aus oder wenden Sie sich an unseren technischen Support, um weitere Unterstützung zu erhalten."
Reply can be in German as well.