The Backend Worker Process error message usually appears because more than one copy of SpiderOak is trying to run at the same time. When SpiderOak is running, there are three separate SpiderOak processes which run in the background. Sometimes when SpiderOak closes, not all of these processes terminate correctly. If any of these processes are still running when you try to reopen SpiderOak, you'll see the Backend Worker Process message.
All you need to do is close the copy of SpiderOak you have currently running, then open a list of processes that are running currently. (On Windows, you can do this by pressing Control-Alt-Delete to bring up the Task Manager; on Mac, open the Activity Monitor from Finder.) If you see any processes named "SpiderOak" running, close them. Now you can reopen SpiderOak and you will no longer see the "backend worker process" error message.
If you are receiving this message each time you start SpiderOak, this is probably because you have set SpiderOak to start automatically from more than one location. Please try disabling the option to start automatically in SpiderOak under "Preferences" and double-check the Startup folder on your operating system.