What options for clean install of MacOS High Sierra after permanent error message?
I finally made it work. It must have had something to do with an encryption I had set up for the FusionDrive previously. 
SOLUTION:
I deleted the Logical Volume Group which resulted in 2 separate 'untitled' drives, i.e. one SSD and one larger HDD drive. I then combined those drives again to form the FusionDrive. After that, I installed the original OS X Mountain Lion via web recovery, installed the High Sierra upgrade via the App Store, and then went back to the recovery mode to erase the volume again and install the High Sierra OS from there. Now I have a clean High Sierra preinstalled iMac, as intended.
I would like to use this opportunity to thank all of you for your support, which was extremely helpful for me in finding the ultimate problem solution!!