To follow up on my own post, I think I figured it out. — This looks like a small bug in the firmware.
Here is how to reproduce the issue:
* Go to “Edit Options/Weather and Season”
* Set the “Weather Adjustment Method” to “Manual” — This will enable the “% Watering” input field.
* Click the “% Watering” input field, set the percentage to 0. Hit “Submit” in the popup dialog. Do not save your settings yet.
* Change the “Weather Adjustment Method” to “Auto Rain Delay”. Confirm a duration of 24 hours.
* You’ll notice that “% Watering” is greyed out again, but reads “0”.
* Save options by hitting “Submit”.
When you do this, back on the main page you will see “Water Level: 0%”. — This prevents any program with “Use Weather Adjustment” checked from running.
To get out to this state, just go through the same steps as outlined above, but set “% Watering” to 100 instead of 0.