I need some help here.
The the main character flashes/turns invisible for a short moment when I execute fall action/animation?
What could be the cause of that?
Any help is appreciated!
I will take a guess and say that your transition blends are not hooked up, or, the animation is messed up? If its the default fall, then, i would say that the animation blend is bad.
This topic is totally not what I was expecting it to be.
ha ha ha 
Does the player fall down in the game then after a short fall, suddenly then dissapears into thin air? If so, check the Z Kill volume under world settings