iMac gets dark when using in target display mode (TDM) and brightness slider missing!

Hello everyone,

I am using a MacBook Pro (13'', late 2011) and an iMac (27'', late 2012). Both are up to date, running El Capitan 10.11.5. They are connected with a thunderbolt cable in order to use the iMac as a second display (TDM).

Now, when I press CMD+F2 at the iMac, the iMac gets used as a second display as intended, but it goes really dark! It's not black, you can still see the desktop but on a bright day it's really hard to see anything. Now this is pretty annoying, but the second part of the issue makes it a pain in the ***. When the iMac is dark, I always go to the system preferences and open the settings for the displays. I can see a window with the settings for the iMac, but there is no slider for the brightness! There is no way to make it brighter!

The only workaround is to disable the target display mode (CMD+F2) just to enable it again after some seconds. Sometimes I have to enable the TDM two times but sometimes it takes 4 or 5 loops of enabling/disabling just to see the brightness slider!

Here is a screenshot of the settings window without the slider

And this is how it looks like after a couple of loops, and then I can adjust the brightness.

Any ideas why this is happening? Any advice regarding a possible solution? Why is it darkening the iMac? Why is the slider missing?

Best,

Stephan