The sanitary sewer had been working since 1974, without any problems. At first the waste water from the WC and shower was disappearing a bit slower than usual. Nobody was considering it to be any big problem before it completely "packed up".
It should be working like the picture below. Waste water and other waste coming to the sanitary sewer through the "in" pipe and water flowing out from the "out" pipe.
After an inspection of the sewer I noticed it was filled with water above the "out" pipe, which indicated the "out" pipe was blocked. The "out" pipe was opened up and cleaned, using high pressure water cleaner. The water started flowing through the "out" pipe and the level inside the sanitary sewer sank to the normal level. But, this did not explain the fact that the waste water from the house was not going anywhere.
The next thing to do was to dig a trench to get to the "in" pipe".
Close to the sewer I found a broken pipe. Pressure from the ground (2 meter/8 feet gravel in a steep hill) had broken the pipe.
The pipe was replaced with a new one (110 mm.).