This is partitialy solved without knowing the exact cause.
Actions taken:
- Removed Office completely.
- Cleared out the registry from Office leftovers.
- Rebooted
- Installed Office 2010 again instead of 2016.
- Problem gone...
- Still on the same video- and printerdrivers.
This is the result in Word 2010:
Question still remains: what caused the strange letter spacing?
I'm a bit wary to install 2016 again for the time being. I'll look a bit further into this first.