@jingwang0222- By any chance did you see event viewer if any details about error is logged? Please check if any path/file is being used in process and that is not being available for one of your colleague
Object reference not set to an instance of an object . Null reference errors are responsible for a good percentage of all application bugs. They are usually very simple problems caused by not adding additional logic to ensure that objects have valid values before using them.
So probably adding more logs before start of process/step where it fails would give you more details.
Problem solved!
Different users has their own account folder which includes uipath packages inside, the one account who has this error because her account’s uipath activities folder dont have enough activities. So i just copy and paste mine activities folder and replace hers, done.