Picture says it all. If IsOnGround() is false i play the fly animation (for the fall). When i jump on some objects like the urn or the fence, or i jump into barell engine still recognize it as flying (not on the ground). How do I fix this?

Me thinks the objects are too small to fully block the main body and thus giving false return.

You could do a double check.
Check IsOnGround() and player's vertical velocity.

Objects usually tend to gain altitude rapidly, or lose it.

Yeah i was checking the linear velocity value in the console, and it was like 0.01 and building up constantly. I'll try volume cast instead of default ray cast maybe it will help.

maybe the ray trace misses the urn

It seems so, but this is also happening if i get stuck in a barrel cause character is too fat. As i said i'll use the volume cast and report the results later cause my PC is overheated atm.

Maybe collision is on in the attached mesh itself. That can cause some weird fuckery.

^ No its not If it was weird fuckery would happen all the time (this is just for some special cases and i don't want them to happen)

There seems to be a kind of "constant" collision, as if the body of the character was colliding constantly. Is it inside some hidden geometry ?

No it is not. Since i couldn't jump on urn anymore (it catapults me now), i jumped into a barred and fat capsule got stuck (thick one isn't hitting the ground). Length of ray cast result is 8 (since of physics model of the barrel) so it detects it. Either it isn't adding force accurately or there is something else going on.

Well you keep saying "no it is not" like you are so sure, but then wonder if "Something else is going on"

The suggestion I gave you was a solid one. Did you at least check to see if the attached mesh has collision enabled? Is gravity turned off? If it's nothing in the resource editor you got some bad code floating around making your guy get stuck.

I'm saying it cause i am so sure. Character has no collision except character code capsule. Here you can see the picture that rabbit is flying too. Default NA didn't change a thing in RE or code (except adding barrel duh). Barrel has no collision on meshes just physics. Here i attached the rar with the barrel. Extract it over Data folder and go to PhysicsDemo map. Barrel is in the corner so jump in it, turn the dynamic physics debug draw on and you'll see it. This also happens with various objects that don't have dynamic physics (like the urn).

P.S. You cannot use this barrel in you projects

urn has a hole in it? you probably saw that but just in case