This is what the problem was: there is a pipe around the turbos which is responsible for oil return into the engine. Basically oil is pushed out of the engine due to high compression and the powerful turbos, and this pipe is supposed to return the oil into the engine.
This "pipe" was not connected properly and a piece was missing, and therefore oil was slowly but surely leaking into the turbos... From time to time this oil would burn up inside the turbo pipes which is the cause of the bad exhaust smell around the turbo area. (see picture at the top)
If you have this exhaust smell then you should take it to the shop to ensure that this "pipe" is connected properly. If you see any oil around the area then you definitely have this problem.
the turbos on the 335d are very complicated... The turbos are the reason why this engine is so powerful.