Got the same problem, also replaced the encoder with no success,
finally, it was the pulley from the motor, witch was full of wear of parts from the belt rubber, clean the pulley from the motor, and it should work
I know cases when it was needed to replace 3 drum belts.
Use a ironsaw blade to clean the drive roller on the motor, maybe it helps, or increase the torsion on the spring,
I used NOT original Xerox belt with 7 stripes in it, original has 9 can that be a problem?
Yes, it can be a problem. So, use original.