Well, rather than join the unquestioning chorus of “should have checked the cancellation policy”, I just tried a dummy booking at the Sheraton Nuremberg for 17 Sep.

The “Member Flexible Rate” is showing the same price, Eur142, with a 1 day before cancellation penalty.

The may well suggest that the OP inadvertently stumbled upon a rogue rate, and that there is a clear and obvious case for the OP to be able to cancel without penalty under the *proper* T&Cs for this particular rate.