Not sure who ur is.
But if a different load cell amplifier works correctly it follows that the issue must be resolved within your design, layout or software.
You do need to identify the source of the signal that is interfering as the proper solution is based on the root cause.
Your mains supply is 50 Hz? If so induced signal could be the cause. Shielding and grounding would be where I would look next if that is the case.
A different possible source is static from the belt grounding through your sensor input and modulating the input. These are strictly guesses as you have not provided enough information to define which section of your device is the probable culprit.

What does your oscilloscope show? Does connecting it to the input signal point affect the readings? If not what doe you see? The superimposed signal should be your source.

If there is no visible signal try loading or if it can be safely done shorting the inputs. If the signal is still there it is entering your electronics through a path other than the intended signal path. Possibly the power supply connection is allowing it in.
Perhaps even probably it is a radiated signal from the motor its drive or wiring that is being picked up by the board. Here shielding will help but other more drastic fixes may be needed including layout changes on the board or even design changes to improve immunity. It is impossible to choose where to look without more information. Terry