Engine malfunction

Hello Phaeton friends, I’m new to the forum and greet you warmly from nrw. Unfortunately I have a problem with my VW Phaeton 3.0 tdi BJ 2008 with 239ps, mileage 131000km. The problem started about 2 months ago. The flashing light flashed and stood in the display engine malfunction workshop. After a restart everything was ok again, then drove to VW. They probably noticed that I had fuelled probably dirty diesel and thus the fuel filter was closed. Then I also had 5 weeks rest. On Thursday again the same game from the front. I to VW, the car was there again 2 days and then told me, who also don’t know what it is but in any case something with fuel input or so similar. Since the engine had to be opened, they could try to regulate for relatively little money anything on the engine control unit. On the other hand, I could leave the car there for a longer time and could possibly cost 12000 €. I did not do that because I had to be a hundred € a little shocked. The problem was still the same to me today.