Char 160 is the 1 before div $Entity/Coupling_Speed
Did you check the value of $Entity/Coupling_Speed. If empty or 0 the expression will fail.
As Tim says, this could well be a division by 0 error. When you get his error, check the Coupling_Speed attribute is not 0 or empty. It's also worth checking the Entity object, and the Balancing_Grade, Spacer, Membrane_Driver, Hub_Driver, and Transmission_Unit attributes are also not empty.
I would suggest adding a breakpoint just before where you carry out this calculation so you can check all the input values before it is executed.
Good luck!