Shield tablet neverending loop

Hello guys,

Today I've tried flashing my Shield Tablet using Shield Custom ROM guide from by cots from gbatemp.net forums (can't post links).

Despite doing everything carefully, step by step, my Shield is now rebooting forever showing only one screen and rebooting again.

Unfortunately I can't post a picture, because forums rules don't allow me, so I wrote down the text from TWRP:

E:Could not mount /data and unable to find crypto footer.
E:Unable to mount '/data'.
E:Unable to recreate /data/media folder.
Updating partition details...
E:Unable to mount '/data'.
...done
E:Unable to mount storage.
E:Unable to mount /data/media during GUI startup.
Full SELinux support is present.
E:Unable to mount /data/media/TWRP/.twrps when trying to read settings file.
Processing AOSP recovery commands...
E:Unable to mount '/data'
--Wiping Data Partition...
Formatting Cache using make_ext4fs function.
E:Unable to mount '/data'
--Data Partition Wipe Complete!
Done processing script file

aaaaand it reboots.
Pressing "Home" and/or "Back" button at that TWRP screen does nothing, Shield is still rebooting to that screen over and over again.

Help would be greatly appreciated. Any ideas?

Best regards,
Łukasz