Hi Arnd,
Regarding the error message you are seeing, which OpenSprinkler are you using and also which firmware?
The way the location works in the app is the GPS coordinates are saved (and not the weather station name). This means you can put a point anywhere you want, including your home address. Weather Underground will resolve the coordinates to the closest weather station (and if it matches the coordinates of the saved station there will be no ambiguity).
The app shows the location by asking Google to geolocate the coordinates. I use Google because the location names provided by Weather Underground are variable and seem user generated.
Thank you,
Samer