JUser::_load: Unable to load user with id
This error message is generated by the fact that you are trying to import data from RSTickets! into RSTickets!Pro and the users for the respective id no longer exist. In order to fix this issue you will need to run two SQL queries from phpMyAdmin(or a similar database tool):
DELETE FROM jos_rsticketspro_staff_to_department WHERE user_id='your_user_id' DELETE FROM jos_rsticketspro_staff WHERE user_id='your_user_id'
These two queries will delete the records from the two mentioned tables for the specified user id.
