well, now we know how to fix it
This is awesome news! Thanks for investigating 
My system is now up and running as well.
Step-by-step-guide to OliverV’s solution:
- Boot via Berryboot and click on “Edit menu”
- Expand the options menu via “More options” and activate “Console”
- CTRL+ALT+F2 opens the console
- Login with the username “root” (without password)
- The input “chroot_image” tells you, which images are available
- Retype “chroot_image”, followed by the individual image name shown (i.e. “chroot_image NextCloudPi_12-04-17_berryboot.img240”)
- To edit the faulty fstab-config file with the editor nano, type “nano /etc/fstab”
- Change the displayed content according to OliverV’s screenshot above
- CTRL+X closes the windows, remember to accept the changes
- Return to the BerryBoot menu via CTRL+ALT+F1
- A click on “Exit” makes the pi reboot -> There you go!
Mind the fact, that the US-keyboard-layout will be active…