There are two of us working on our game, and it seems like the mass of our player's rigidbody changes depending on who's running it. On my partner's computer and in the first  build, the game ran as expected. On my computer and in the second build, the player is treated as being too heavy to move. As far as we can tell, we both have the same version of the game and all relevant settings are identical. Does anyone know of a way to fix this?

Multiplying everything by deltatime seems to have made everything more consistent, but we still don't really understand why. The only thing I've found is that the frame rates must be very different, but I don't know how that would happen.

Try using FixedUpdate for physics instead of Update, if you are currently using Update