OK, I’ve solved the problem.
When I did the fresh install I immediately updated all of the packages before building OSPi.
It turns out that there is a package (not yet identified) that interferes with the web UI.
I did a fresh install of Raspbian, no updates and then built ospi and it now works.
My mistake was that I had enabled unattended upgrades on my opensprinkler Pi and when it rebooted after the power outage the updates kicked in and I no longer had a working web UI.
Now all I have to do is find out which package is causing this.