If you're running Sophos or any other so-called "anti-virus" or "internet security" software, remove it according to the developer's instructions. Reboot and test.
Otherwise, open the pane in System Preferences. Click the padlock icon in the lower left corner, if necessary, to unlock it. Scroll to the bottom of the list of backup drives and click . Remove the Time Capsule, then add it back. Quit System Preferences. Test.