Solution: My service wasn't started. Because we run services locally, I needed to open the required solution and run the service locally via RG -> Create new instance.
How I do I know about this? My log files are stored in c:/temp. In this location, appeared a new log file and there I found more info about my error. (All the time I was thinking that the details about my error message will enter into my basic log file, but I didn't expect another log file.
So, Reason: web service didn't work.
Solution: Start service localy: Rb -> create new service.