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Have you tried the tape mod on the back of the pcb before doing any permanent mods on the board itself? To plug the hole I would try to do something that is reversible. Backing tape and hot glue to fill in the depression.
Hello Friends, i bought my first MechanicalKeyboard on ebay (it is an Akko ACR59). Sadly, the Key for the Questionmark aint working. I unpinned the key and pushed the button under the regular key (on the board, if you know what i mean), but it just aint working. 
What can i do now. I dont find an option in windows to say that another button should do the questionmark. 
Can someone help. >

It's hotswappable, so you can try removing the switch (presumably it came with a switch puller; this is NOT the same as the keycaps that you already took off, if i am reading your post right) and seeing if it has bent pins/swapping it with a different key's switch.  
There's also probably some AKKO software that will allow you to remap which key does what if all else fails. >

This worked! Thank you! I would have never thought, that it is saved on the single buttons. Holy... Man Thanks! >

How would one fill a hole on a PCB? 
I bought the Maestro TKL with Cherry MX browns by Archisite, a Japanese company.
One problem is the B key feels really clacky and feels like somethings stuck inside. Also only the B key switch is upside down (The Cherry sign is on the bottom). They probably reused the PCB for Quattro, another of their keyboards with a pointing stick, that's why.
I changed keycaps and soldered another brown switch on, it got better but still feels clicky.
But the Quattro keyboard doesn't have this problem, so I suspect it's the gaping hole that they left on the PCB that is the problem.
How would one plug the hole on a PCB board?
It's a circle with a diameter of about 7mm I guess.