Thanks for the hint and yes, I am running a two Monitor setup and Cura opened on the right side monitor.
The Window was visible but completely frozen and unresponsive, similar to what TMan4557 described in a previous post here:
Not sure how I got there in the first place, but I was able to reproduce this issue with Cura 5.6.0 by adding the window lines back into the config file.
As already mentioned, deleting the window lines in the config resolved this issue and didn't happen again after. It seems that there might be two different issues described by previous users:
1) Cura does not appear to be opened (window not visible)
2) Cura freezes and is stuck with a white screen