thanks for providing instructions on resetting the root password here
Unfortunately, the command chroot /mnt/sysimage failed on my install. /mnt/sysimage was not mounted - possibly because I had a custom partitioning.
by typing lsblk I could find my root drive
mount /dev/sda3 /mnt/sysimage mounted it.
Then I had to run
in order to chroot into it.
Maybe this helps someone some time.
Would be interested to hear, whether this issue is specific to a custom partitioning and whether there is an easy fix.