[PVE-User] grub-install error: unknown filesystem, won't boot anymore

[arjenvanweelden at gmail.com]

Dear readers,

My up-to-date (no-subscription) Proxmox installation at home stopped
booting properly. GRUB complains at boot about:
  error: no such device: <hex string>.
  error: unknown filesystem.

I got my system booting again using a new installation of Proxmox on
a USB-stick, because Rescue Boot option of the installation ISO did not
work for me. Fortunately, my 4-way mirror rpool is just fine (as are
the other ZFS pools), but grub-install, grub-probe, and insmod normal
at GRUB prompt keep returning "error: unknown filesystem".

Last thing I did to my rpool was 'zfs set dnodesize=auto rpool' as
suggested on the ZFS Tips and Tricks official Wiki-page.
Reverting to dnodesize=legacy did not fix the GRUB boot issue.

Currently, my system is booting partially from the USB-stick and
partially from my original rpool, which works but is not ideal.
Does someone recognize this problem? Does anyone know a fix?

kind regards, Arjen