According to newspaper:
There were several disaster of 737 in the US that had unknown cause in nineties. Finally low temperature problem with hydraulic servosystem of elevator was found.
In time of investigation of Kazan disaster, MAK sent a question to FAA/Boeing if related hydraulic part of involved aircraft was replaced or not. Till now they have got no concrete answer, so they can not say, what 737 are safe and what not. Maybe they try to force Boeing to answer.
This episode is little confusing but by my opinion, investigators of MAK are very competent