1) Check the physical path Service Desk is looking for the file by going to the Administration tab-> Attachment Library-> Repositories
Select the proper Repository and then right click on the file which is generating the error and select View Details.
Verify the physical path set for the file and make sure it exists in that exact location.
2) Check the Attachment Servlet Path set for the server by going to the Administration tab-> System-> Servers
Select the appropriate server from the Server List and verify that the Attachment Servlet Path is set correctly.
This error can also occur when Service Desk is configured for https, but the Attachment path is set to http.
3) Verify that the the rep_daemon process is running by logging into the server which Service Desk is running on.
Open a command prompt and run the following: pdm_status
This command will give a status of each Daemon Service Desk is utilizing.
Check if the rep_daemon process is running, if not then try to run: pdm_d_refresh which is the command to start a failed daemon.
If that command does not work then a recycle of Service Desk will be required.