You’ve done the first things;
Tape guides.
Encoder/tach coupling (motor and encoder). Assuming the encoder is on the motor end shaft.
Tach/encoder wiring in the drive unit tight.

This happens anywhere in the hoistway in the down direction? High speed or roll off to high speed (acceleration profile)?

No other faults whatsoever?
Can you set the RVU to capture the error 24 and then review the recording?