I think you need ubi0:root, Not ubi2:root.
Thanks, you've got a point, but still I had no success.
mount -t ubifs ubi0:root /mnt/ubi
mount: ismeretlen fájlrendszertípus: "ubifs"
Meanwhile, I have discovered that dmesg contains some useful(?) information on the error:
UBIFS error (pid 1874): ubifs_get_sb: cannot open "ubi0:root", error -19
Some googling suggests that I may got the volume name wrong, I will look into that.
Update: I figured it out!
The missing step was an
ubimkvol /dev/ubi0 -m -N rootfs