Help to extract KGB file

Guys I'm doing a reverse engineering CTF and got stuck with a unknown file type.
After analysing it on a hexeditor I figured the magic numbers were pointing to KGB2, but none of the KGB versions could extract the files.

I already enabled older .NET versions, my guess is the file is just corrupt.

Have anyone seen something like this on CTFs? I need some advice.

Also, the main goal of the challange is to build a keyfile to access the file (inside the KGB) contents.