I am getting a 01 20 fault on every run in both directions. At the top landing it goes on to final. I had several 00 81 faults in error log. I can't find any info on 01 20 error. Any suggestions?

Update: I changed parameter 6-3 from 18 14 to 18 00 and that seems to have eliminated the problem. This parameter is for rollback. Anyone able to explain some of the technical details of this?

Sounds like you got it fixed.
Just some things to the check.
Double check load weighing.
Double check car and counter weight balance.
Had some jobs with improperly balanced weight frames.
Also make sure you aren't slipping traction at top landing.