I’ve been having similar problem also recently (Native PC x86-64) on my Z83, latest 6.21.1 version installed. Something is changing/corrupting fstab, so a reboot results in a broken system if fstab is bad. I did a complete re-install yesterday and within an hour after setting everything back up noticed that fstab was changed again after the attached USB drive for my FTP server was unmounted. I saved a backup of fstab just prior, so I overwrote the bad one and then made fstab read-only. Hopefully this temporary fix works until the root cause is found.
https://dietpi.com/forum/t/help-dietpi-completely-fubard/2986/1