This is not a bug.
As I announced with the "Whats new" dialog, Diarium now uses a new Weather provider (Apple Weather).
The old weather provider (Dark Sky) was bought by Apple and will be shut down completely in 2 months. So I had no other option than to switch. Apple Weather does not have the weather text.
My personal opinion: The weather text was inaccurate most of the time anyways and the new weather data features the daily min temperature, which was not included in the old weather.