Since a week or so I’m having the same difficulties. The water percentage is stuck at a certain value and no last successful weather call is shown on the weather diagnostics.
Restarting the OpenSprinkler service (I run OS on a Raspi) immediately fixes it and updates the water percentage, but it gets stuck again after a few days.
Sure, I could restart the service once a day via cron, but that is only a dirty workaround…
Cheers,
Michael