Trying to run the game:
- via gog galaxy
- Win 8.1 Ent
- game version is in the attachment

The game loads up to 50% and then is not responding. I tried running it in lowest resolution without full screen via il2fb.exe, but the result is the same. I also reinstalled it, but without effect.

What can I do?

Exactly the same problem here and same OS. I notice you posted this a week ago, and still have zero response. It looks like we should not try holding our breaths till someone offers a solution.
After launcing the game, there is no response from the PC whatsoever, for at least three minutes. But then up does pop the title screen - at 50% loading complete! And there it sits. Though it does continue past 50% if you wait, for a, quite frankly, unnacceptable amount of time (at least 10 minutes). Then the old sepia effect video cuts in. <Esc> to exit does nothing. All keyboard controls and mouse controls are kaput. If you try <alt-tab> the game just crashes outright. I has not done that on any of my older PCs in the past, and loaded quickly on those older PCs.
No <alt-tab> means I can no longer review the aircraft guide whilst in mid-play anymore.
I've owned copies of "IL2" since 2001, and owned this GOG version since 2013. I periodically keep coming back for more. But this time no dice whatesoever, it now seems a completely broken product.

I remember the "GOG promise" from years ago when GOG was, well "GOG". it simply went "It works, we promise".
That is also now simply untrue.

Don't know if this helps but I did manually change the resolution settings in the conf.ini file to match those my desktop is set to. It made slight improvements. The game still takes a VERY long time to load - and once running, still looks bad - though my GPU could be to blame there.

From what ive been able to narrow down, IL-2 1946 is completely incompatible with any recent updates of windows 10 and what I assume to be 8 as well, no one will resolve this, trust me, anyone who runs this sim daily is on windows 7. id try running this through a virtual machine or on a linux machine if i were you.

Im also very distraught by this, as before i totally fried my cpu and replaced it, I was running this sim daily on my windows 10 rig. the lack of support that the IL-2 community has given is disgusting. this game has totally been forgotten to time.