@Marie Still no joy. The support desk at openweathermap.org is really letting me down. Since I did not get a response for weeks I created at new ticket last week, but I did not even get an entry confirmation. Maybe these guys are close to going bust, I don’t know.
To mitigate the problem I am now working on a blacklisting feature which can be used to ignore outliers. Apparently, the outliers remain constant for a long like your “14.9°C” which is “7°C” in my case (seeing this for months now). I’ll also add a feature to detect potential outliers. This should make it a little bit easier to configure the blacklisting feature.