This error is caused when you have multiple instances or thread of Song Surgeon open.

You may only see one open instance of Song Surgeon on your screen, but in the Task Manager you wlll see these other open instances.

To solve this problem go to the Task Manager
Then go to the Details tab.
In this details window you will see a long list of processes running on your machine. In this list find ALL of the Song Surgeon processes that exist and kill them.
Once they are all killed, then close your Task Manager, and go back to the SS icon and try opening the program again.

The program should open and this error should no longer appear.