An update:
I’ve tried a fresh install on a different RPi (Raspbian Lite) and I have the same problem!
The WebUI is not available and nothing is listening on port 8080.
So it seems this is not an issue with my OpenSprinkler/RPi
PS: Is there any way to change the post title to reflect that this is really a non-functioning WebUI and not an issue with a power outage?