Alright, fixed it. Got me a new internal Displayport cable. Somehow i‘ve must have broken it.
Swapping out the hard disk was ok. What happened after this:
imac‘s display stayed dark although Mac was working and booting up (fans running normally and hard disk working i could heat that) had no choice to shut down the hard way, i.e. holding power button
I suspected the displayport cable wasn’t properly in its socket and it wasn’t. Fixed that.
But then: imac won’t turn on. Tried SMC and NVRAM resets. First fans came back and then chime. But still the display is dark. Chimes comes but This time hard disk seems to do nothing at all.
Why my iMac wasn’t turning on I found out the next morning by pure luck: when it’s running and I’m tilting it back it shuts down as if something inside isn’t working only if Mac’s in a particular angle.
So question: anything I missed here or something defective?