My workaround using the Meteoware Plus service to send my Netatmo weather data to Weather Underground is working with interruptions. However, my free WU API key is working as I can connect my OpenSprinkler system as well to a nearby non-Netatmo WU PWS. I hope that this will be possible at least for the remaining garden season.
Operating the OpenSprinkler with the Zimmerman method works best, if yesterdays mean temperature, total precipitation and mean humidity are available and can be used as a stable input. In my opinion it makes no sense to e.g. use the current temperature as input for the Zimmerman calculation. It is also a problem if any forecasted precipitation for today is added. There are very often rainfalls forecasted, which will never take place during the ongoing day.