Thanks, UFHH01,
I found there are so many longtasks params and in long tasks, therefore, I delete all of them from the record.
I then move customers to another plan and delete the original plan. And create the plan again and move them back.
No more sync problem message anymore.
From my understanding, since the primary key should be unique, therefore, deleting records in longtasks and longtakparams tables should resolve the issues.