When I installed High Sierra, I enabled the new Apple File System for the SSD. When I downgraded to Sierra via USB Stick and clean install, I formatted the Disk to HFS+, but the APFS Partition remained as well showing the same size which was really weird. So although I formatted the disk I had now 2 Partitions. So I installed it on the newly formated HFS+ Partition - that's what I though at least…
BUT after the install finished, the Mac Mini rebooted from the stick because there was no system Disk available which I found out later, when I opened Disk Utility. The APFS Partition as well as the HFS+ Partitions were was now gone and the whole Disk was basically just a block of unallocated Space (besides the 200 MB Recovery).
So I created a HFS+ Journaled Partition again and then, after the 2nd round of reinstalling macOS Sierra, everything was finally back o normal again. I cannot imagine doing this on my Hackintosh. Totally confusing.