Export and Import lists from one server to another - Workflow associated with lists not allowing to modify the contents of list

Hi,

I have migrated the SharePoint Lists using Export command from one server (Source Server) and imported using Import command to other server (Destination Server).

All lists are working as expected, however in one list in the source server have some workflow. 

The list associated with workflow is ticket system. After importing it to the destination server, it is not allowing to modify the existing tickets. Throws an error: "The user does not exist or is not unique."

However I can create new ticket and modify the created ticket, but not the tickets which were imported from the source server.

Please suggest

Thanks.