And when I am trying to load the game with FNV4GB, I get another error - "Unable set current dir to game dir". Strangely it happened to me when I got CTD with NVAC installed. I tried every suggested method from google, like deleting ClientRegistry.blob, verifying game cache, re-installing both NVSE and NVAC. Still no idea how to fix this.

The Same thing happened to me many times, but quite a while ago so I'm rusty on the solution, but you should try removing the game executable and then verify your game. I hope that works for you.
Also remove the exe from the fnv4gb folder and copy and paste the newly downloaded one in.

tru3th wrote:

The Same thing happened to me many times, but quite a while ago so I'm rusty on the solution, but you should try removing the game executable and then verify your game. I hope that works for you.
Also remove the exe from the fnv4gb folder and copy and paste the newly downloaded one in.

Thanks for the response, but sadly it didn't help =\

I'd back up my data folders and reinstall NV, personally. Generally the only way to fix these kind of issues, when it gets to be problem-on-problem I mean.

Solved! If anyone is interested or has the same problem, I have stopped participating in Steam's beta, restarted it and now everything is okay.
Locked.