@MikeyH Hi Mike,
This error is related to the timing issue of the Modbus Driver. It means that the Modbus Driver exceeded the allotted time 800x from the executive start for some reason. In your case, I would inspect the timing of the whole project and also the diagnostics of tasks related to the Modbus driver.
Without knowledge of your project, communication frame and timing, I'm not able to provide more details.
Cheers, Tomas