I had the same issue and after all I confirm that the problem is exclusively with armbianEnv.txt being corrupted. I spent many hours trying to figure out how to fix it and fortunately it was easy.
Steps:
1) burn a SD card with the same Armbian version as installed in your emmc
2) boot from the SD
3) mount the emmc:
mkdir /mnt/emmc
mount /dev/mmcblk1p1 /mnt/emmc
4) copy the armbianEnv.txt from SD card to Emmc
cp /boot/armbianEnv.txt /mnt/emmc/boot/armbianEnv.txt
Important:
* Armbian 21 moved armbianEnv.txt from /etc to /boot
* Armbian 21 is using UUID for rootdev configuration
* blkid is the same for emmc and sd
