This error message can be caused by any of several different things. It most commonly appears when more than one instance of SpiderOak is trying to run at the same time. Sometimes when SpiderOak closes, not all of its processes terminate correctly. If any of these processes are still running when you try to reopen SpiderOak, you'll see this message.
In this case, close the instance of SpiderOak currently running, then open a process manager. 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. On Linux, use your preferred process manager.) If you see any processes named "SpiderOak" running, kill them. Now you can reopen SpiderOak and you should no longer see the this error.
If you see this error each time you turn on or log in to your computer, this is probably because SpiderOak is set in more than one location to start automatically. If two instances start simultaneously, the first will be able to connect to the backend worker process, but the second will be unable to create a backend worker process and will exit with this message. Disable the option to start automatically in SpiderOak under "Preferences" and double check the startup folder, Registry, or similar auto-launch mechanisms of your operating system.
Confirm that you have added SpiderOak to the whitelist in any security software you have running. Although this is most often an issue on Windows, we have sometimes seen this on Mac and Linux as well.
In SpiderOak's preferences, go to the Backup section, uncheck 'Enable Preview Generation', save the change, and restart the application. There is a known issue where attempting to generate previews for certain large or corrupt images can crash SpiderOak.
Consider if especially large files (such as encrypted disk image files or virtual machine images) have recently been added to your backup set. A large file that consists of all new, unrelated data blocks can take a long time to compress and encrypt, which can make the application believe that the backend process has died.
In version 6.0 of SpiderOakONE, attempting to empty the deleted items bin would also cause a backend worker process error. This has been fixed in more recent versions, so ensure that you are running the most current version available from the download page.