Hey, I just posted my question yesterday, but I was naive to receive some useful advices. So I had to figure out myself. Here we go...
As I wrote, my game crashed while recording and recorded video just got corrupted. Yeah, I know, it sometimes closes up recording automatically, but in this case it didn't happen. I couldn't play it, convert it, anything. VLC just played audio, but it was everything, there was problem with that FICV codec Mirillis uses. So I went with VirtualDub. It started to reconstruct blocks, but after that program crashed. Then I was really hopeless. I tried a lot of programs (DivFix++, Repair Avi), but none helped me a bit. I was totally desperate, cause the video was my best I recorded as my let's play videos and I didn't want to lose it. I even went in HEX and tried to manually add headers (that's the cause, of course), but it didn't help. Fortunately, I found really simple solution. So, let's start!
1. Record some sample from random game using Action's AVI option.
2. Get Avi Repair Tool (really small application, just one executable).
3. Load your sample as source file and corrupted file as corrupted one.
4. In a second, your corrupted AVI will be orewritten (can be seen in a date), but still unreadable.
5. Go to VirtualDub and load your modified corrupted AVI, it'll start to reconstruct.
6. Your AVI should be loaded and playable (no more crashing), then just save it as AVI (but before that go to Video and check Direct stream copy).
As you can see, it's really simple! At first, VirtualDub didn't work, but after modifying missing headers from sample AVI, it works like a charm! Discovering this method cost me one whole evening and morning, but the result is priceless.
But there are also several problems, where one could be, that it'll not recover completely. If you extract only sound (which can be made with any converter), it'll have it's original video length, but after recovery, it can lose some of it's parts (like mine 1,5 hour long video became only 26 minutes long). It's something I'm finding solution for now, so I'll add new info, if I'll figure out.
I hope this will help everybody, who will have or just had the same problem.
Unfortunately, Mirillis support is just like any other supports out there, they can't really not help and if they try, they have just nonsensical questions leading to "hardware problems" and pointing out, that this is just your problem. So it's about us, users, to find out method to save our videos.