by Zoran Jeremic.
After several days of trying to figure out where the problem is, I managed to solve this. In one of the methods used within service implementation I had echo which outputs some values which I used while I was testing the service. I forgot to delete it, and it has been returned as a part of the service response, which prevented service client to read returned value properly. Unfortunately, there was no indication that something is wrong with service itself, so I didn't pay too much attention on this.
Zoran