HB wrote:I assume the gauge orifice is blocked.
Thank you for your replies. That is odd. There is nothing I could think of, that could block the gauge orifice.

I will unscrew it and have a look again.
EDIT 1: I have unscrewed the manometer, and the hole is clean. When I screwed it back in and put it into the machine, there was again nothing. Then I waited for 10 seconds, and finally it moved. But it
stopped at 3.5 bars after like 2-3 seconds slow moving from zero and then the seal between the group and portafilter began to leak.

That is odd, because when I do backflushes, it does not leak. I have tried with basket, without basket, fixing the portafilter as strong as I could (pushing the handle to the right). Still leaks and I can't get above 3.5 bars.
What could be wrong now?
EDIT 2: I tried to put the rubber disk (as for backflush) in a basket with a DIY thingy, and it still leaks. But when it is in another basket—it does not (kept it under pressure for 20+ seconds). So I guess there could be a problem with the portafilter itself? I will try to screw my DIY portafilter gauge on another portafilter after I get home from work.
Or do you have any other ideas?