Tried two separate times to upgrade from 3-26-18's build which resulted in a non-responsive device.
Pulled the USB thumb drive and reimaged back to 3-26-18's build and it booted up with little issue.
I used https://etcher.io/ to write the firmware image to a new USB stick, Did this a few times on different sticks, and no problems yet, using 4-6-18 image now on ERLight-PoE5
After reading your post, I decided to give it a second/third go. This time ended up erasing and set to defaults, upgraded through web interface with the previous build (r35667). Booted up with no issues. Then moved onto the more recent build (r35681) and from there it updated and rebooted with no issues.
So thanks for posting your reply. I'm using a 3rd party rather than the OEM thumb drive, so not too sure if that had contributed to the issue. Or if it was simply a PEBKAC error.