As IB does not allow making or taking delivery, could it be that it is not allowed to open new positions after the first notice date (which was on 31/07 for the MGCQ0 contract)?
Edit: More info here https://www.interactivebrokers.com/en/index.php?f=deliveryExerciseActions&p=physical
Not only that, but any existing position should be closed one day before IB's "futures cutoff date" otherwise they liquidate it.
Corn Futures (ZC) example:
https://www.interactivebrokers.com/en/index.php?f=1567&p=examples