First of all, thank you for all of your responses!
pretty sure "Bitcoin Install Dir" should be C:\Program Files\Bitcoin
Pretty sure it has to be a custom directory since I'm using the portable version of Bitcoin Core. I thought it has to be the directory where Armory can be found. Now I know it has to point to the Bitcoin Core directory, but it is obsolet because you wrote that I should deactivate the "Let armory run ..." option. Now I use the armoryqt.conf file as suggested by you:
satoshi-datadir="H:\Blockchain Core"
datadir="C:\Users\Andreas\AppData\Roaming\Armory"
This is how Armory looks like, now (still offline

):
http://fs1.directupload.net/images/180217/kazl7xwm.pngThis is the armory settings dialog as it looks now:
http://fs1.directupload.net/images/180217/7mg4urkm.png