@thermokarst good idea to split this topic off.
Thanks for your step by step instructions (it's a bit embarrassing that I actually need it). So one thing I was not aware of was that the Shared_Folder gets a "sf" prefix and is located in /media. Ok, fine.
However, the shared directories in /media are empty. So unfortunately they do not show my expected files of the host system and when I create a file in ubuntu it does not pop up either...