Import and Mount ZFS Pool: "dataset does not exist"

I am trying to rescue data from a Truenas / FreeNas Pool via Ubuntu. Originally in Truenas from one day to another my Pool wasnt accessible anymore (see this post). Actually the system got in a reboot loop, but that isnt the point.

Now in Ubuntu I do see the pool "RaidPool" via

sudo lsblk -o NAME,FSTYPE,SIZE,MOUNTPOINT,LABEL

Answer ist:

NAME FSTYPE SIZE MOUNTPOINT LABEL loop0 squashfs 55,5M /snap/core18/1988
loop1 squashfs 64,8M /snap/gtk-common-themes/1514 loop2 squashfs 31,1M /snap/snapd/11036
loop3 squashfs 219M /snap/gnome-3-34-1804/66
loop4 squashfs 32,3M /snap/snapd/11107
loop5 squashfs 51M /snap/snap-store/518
sda zfs_member 3,7T
├─sda1 2G
└─sda2 zfs_member 3,7T RaidPool sdb zfs_member 3,7T
├─sdb1 2G
└─sdb2 zfs_member 3,7T RaidPool sdc 111,8G
├─sdc1 vfat 512M /boot/efi
├─sdc2 1K
└─sdc5 ext4 111,3G /

Try No 1:

zpool import RaidPool

Answer:

cannot open 'RaidPool': dataset does not exist

Try No 2:

sudo zpool import RaidPool

Answer:

cannot import 'RaidPool': pool was previously in use from another system.
Last accessed by (hostid=99d0d700) at Sun Jan 17 02:50:30 2021

Try No 3-5 (all with same answer):

sudo zpool import -f RaidPool
sudo zpool import -o readonly=on RaidPool
sudo zpool import -o readonly=on -f RaidPool

Answer: Nothing happens, the curser just goes on and off. I dont hear the harddrives, I gues nothing is happening from this point, so I close the Terminal after 10 min.

Finally via Ubuntu UI I do see the RaidPool, but when opening it I get the message: "Error mounting /dev/sda2 at /media/chd/RaidPool: unknown filesystem type 'zfs_Member'

Any ideas, how to import or just read the data in that pool?

Thanks very much upfront!

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