Have you experienced a System Error dialog warning on (OP) such as this? Do not worry, we have just the right fix for it.
To resolve this issue you will need to open System Configuration then select OP.
Next step is to click on the Applications tab. In the User applications box, head over to the Application is not available column and find the Maintenance checkbox.
Select the Maintenance checkbox so the checkbox is ticked on, as shown in the image below. This will solve the issue with the warning dialog.