To fix this:
- Acknowledge the message by clicking "OK"
- Go to File>Options>Calendar
- in "Word time" change the End Time to any time between the shown Start Time and 23:59 (i.e. 23:30)
- click "OK"
This change can be made "on the fly" without the need to restart Outlook, the PC, GMetrix or the Exam, depending on when you encounter the issue.