Im thinking the rollers arent seated properly . Might be missing the spacers that hold the rollers in place properly . Try rolling the rollers by hand first and see if they stay in line
You might go online and see if you can find a parts diagram. If so you can check yours against the diagram to see if you have all the parts and they are in the right places.
Also double check that you have the right belt and it is installed correctly.
With the belt installed but before you install the cover. Wiggle the roller and see if it may be too loose, if so, look at the roller mounts to see if they are worn - both the roller and the place where they mount.
Look at the pulley on the belt drive motor. Look for a buildup of rubber in the groove. If so, clean it out. Check for a vibration in the motor shaft. Shafts in electric motors sometimes has in/out play but that is minimal. Any side to side play indicates worn shaft bearings. Also check for wear in the grooves of the pulled that have become sharp - use a file or sandpaper to smooth them evenly.
Check the cover for dents or dings that may be contacting the roller. Check for obstructions that might hit the belt or roller only when in operating position.
Set your floor height to a higher clearance with the floor.
As a truck mechanic I learned how to troubleshoot fan belt failures. Look at the old belts and see if there is a wear pattern to the break.
Good luck.
I take my vacuum cleaner completely apart 1x a year and clean and inspect it. Its 4.5 years old and never broke a belt.