Usually this message appears when there is an endless loop in the application.
OS checks the time the controller passes through program end. If this time is bigger than predefined in watchdog different got different PLC models, (but ways bigger than ordinary scan), it stops the controller and outputs such message.
The most "natural" reason for such message, is program loop without clear condition to exit.
When this message apears you will see also LDR error number.
This number will help you find the subroutine that is causing this issue.
To go to this subroutine follow the next steps.