Is a Thunderbolt cable usable between a MacBook and a non-Apple external display? A Thunderbolt cable has a Thunderbolt-connector on every end. A Thunderbolt-connector follows the Mini Displayport specifications and should be usable between Apple devices as such.

When I connect a Thunderbolt cable from my 2015 MacBook Pro (i7 4890k, AMD M370X, 16 GB, 512 GB) to my 4K-display (Acer S277HK), the display powers down immediately and I can't power it back up again until I've removed the Thunderbolt cable. Is this normal behaviour? Is the problem in the cable, the display or the display's mini displayport connector? Or are Thunderbolt cables generally not supposed to function as Mini Displayport to Mini Displayport cables?
No.  You cannot use a Thunderbolt cable to connect to a mini Display Port (mDP) monitor.

The reason you are seeing the monitor shut off is because you are sending it Thunderbolt AND Display Port signaling.  Remember, Display Port is, for a lack of a better word, embedded within the Thunderbolt signal.



Just using pin 1 for example, the Display is expecting ground while TB is sending HV Input.  

This is why you need a mini Display port cable when hooking up a monitor and a Thunderbolt cable when hooking up a Thunderbolt device.