Hi,

Thanks for your support! I didn't have time to answer, but I tried almost everything

This part of the extruder is exactly the same as on your picture. Also I've received new one from German RepRap and it didn't help, but I managed to figure out possible cause - diameter of the filament differs from 1.7mm to 2.2mm!
After one of jams I had to cut the filament and as usual tried to push it through the tube towards nozzle, but I wasn't able to do so. It just stuck in the middle. then I decided to measure the diameter and I was a bit shocked when I saw 2.2mm. I have 2 different filaments from German RepRap and both of them have the same issue.
On the other side, I checked 2 times cheaper filament from 3d factories and it is just perfect.

Solution with fan works great. I have connected the fan to the connector at the bottom of printer and it blows constantly.
By the way, I've found one more problem with the printer. The fan which supposed to blow to board (the one with the bluse light) was tighten too hard and was not working. :-\ Don't know how it has passed QA.

Best regards,
Evgeny