I Also have an ENS202EXT that was on 18.06.1. and I ran into the same problem trying to upgrade to 18.06.4. Found this thread, but without the answer. I found the answer on another thread shortly afterward and wanted to share.
Basically, this is a bug that only exists in 18.06.1 for the ar71xx branch it seems...
as the person who figured it out describes, you have to do the following before upgrading:
(first, I recommend factory reset / firstboot and reboot beforehand)
create the lock file for uboot env: touch /var/lock/fw_printenv.lock
Make the changes for platform.sh (approx. line 10) as seen in this commit: nano /lib/upgrade/platform.sh RAMFS_COPY_DATA='/lib/ar71xx.sh /etc/fw_env.config /var/lock/fw_printenv.lock' RAMFS_COPY_BIN='nandwrite fw_printenv fw_setenv'
Load the new image however you like (to /tmp) and upgrade (not keeping config) sysupgrade -v -n /tmp/*.bin
Actually I forgot to update this thread as I found out an easy way (in my opinion) to upgrade those EnGenius units, which require we to force them to enter into "Failsafe mode", can be get through an easy way.
After many failed attempts and frustation of nothing works to upgrade or downgrade from 18.06.1 to any other version, I found this following command as working great for those who are stuck at this version. From what I understand, it forces unit to write directly at MTD linux partition, through this way:
mtd -r write /tmp/original_firmware.bin linux
On my case, after doing this, the unit reboot and entered into failsafe mode as pictured below:
