Normally, this error is caused by the Online Backup config files becoming corrupted. This can happen if the computer unexpectedly shuts down while writing to the config files.
The solution is to delete the config files.
Start the terminal on your computer by spot-searching for "Terminal".
Type the following lines in the terminal, one at a time, enter-clicking after each line that is pasted:
rm -rf ~/Library/Application\ Support/StoregateOnlineBackup/
rm -rf ~/Library/Caches/se.storegate.StoregateOnlineBackup/
rm -f ~/Library/Preferences/se.storegate.StoregateOnlineBackup.plist
Then restart the computer, important to do as Mac will cache the old config file otherwise.
Then set up a new backup.