This error is due to the connection to Sage 50 has become de-authorized; so the connection becomes invalid.
To fix this issue you would need to follow these steps:
Navigate to the integrations page of Receipt Bank and where the current integration is displayed (Sage 50) and click on the blue 'change' button
On the next screen click on the red 'disable integration' button
Then the connector needs to be uninstalled. This can be done by finding the connector in the task bar, right-clicking on it and selecting uninstall
Finally Sage 50 needs to be re-integrated by following the instructions here.