Player stopping or jumping out of nowhere!

Hello,

i'm trying to do an infinite runner for learning and i've encountered a problem: my player(a square) is thrown in the air and, if I check the fixed angle property, it stops instead of being thrown. I checked all the box colliders, the platformManager script, the physics material of the player and platforms(both with 0 of bounciness and 0 of friction) and I haven't found anything. The platformManager is instantiating new platforms perfectly aligned with the old ones, I've checked. I have also fixed the Y position and made my player "float in the air" and it still collides and stops or slows down. Do you guys know what can be the problem?

If you want to check the game out here it is: https://www.dropbox.com/s/welb3qcjin0ooaj/SpaceShooterLearning.rar?dl=0