In order to get this working again for everyone, I made a list of what I found out to get your Fusion drive up and running again on a macOS Extended (Journaled) iMac running High Sierra. With great help from this forum!
1. The first thing you should do is unenroll your machine from the Beta High Sierra software https://beta.apple.com/sp/betaprogram/unenroll
2. Make sure you have a good backup, preferably a Time Machine backup, so you will get all your settings etc like you want them.
3. Have your iPhone, iPad or whatever internet device at hand and go to https://beta.apple.com/sp/betaprogram/apfsfusion On that page scroll down to Using Internet Recovery. And go to this page where my/this explanation is https://forums.developer.apple.com/message/272368
4. Restart your computer and use Internet Recovery (hold Option + Command + R keys).
5. Make sure you have a keyboard and mouse with wires! Because Bluetooth won’t work for a while.
6. Go through the steps and commands as listed on https://beta.apple.com/sp/betaprogram/apfsfusion the Using Internet Recovery part. I know they seem a bit difficult, but hey, your Mac is stuck anyway. Just make sure you type those commands exactly the same even the spaces are important!
7. The first command you will type is “diskutil list internal” now you see a list of disks connected to your Mac. Look for the internal disks (disk0 and disk1 in the example, but they could be different numbers in your case!) and write them down. They are the ones labeled (internal) and (internal, physical).
8. In step 5 use “diskutil unmount force /Volumes/APFS_Fusion_Volume” command, just to be sure.
9. In step 6 and further make sure you use the disk numbers you wrote down!
10. If all goes well you can choose the "Reinstall macOS" option and select the "CoreStorage Fusion" volume as the target. But I didn’t choose to migrate my data just yet and created a new user with a different name as the name I had on the iMac.
11. After starting up (as a new user) with a clean and fresh iMac, I first checked if it was HFS+ and the Public Beta option wasn’t activated (a bit unnecessary but after so many fails I just had to know).
12. Now I updated to the latest version of High Sierra and made a new (super)user with the same name and password I had on my 'old' account.
13. I restarted, logged in to my ‘old’ account, removed the one I made and used Migration Assistant to get my TM backup on my iMac again.
14. After several hours I had my iMac back again, with everything in place and blazing fast! 🙂
Note: If anybody sees a fault in my steps let me know, I will correct my faults so nobody will get into trouble…
Cheers and hope everybody gets their Fusion iMacs back!