Issues with Stacking Ethernet shield and USB Host shield
Both shields are SPI devices, and both shields seem to be hard-wired to use D10 as the chip select line.

In order for you to use both shields at once you will have to either modify one of them or connect the shield via jumper wires.

You will need to change the chip select connection on one of the shields from D10 to some other pin that is not currently used, and then modify your software (or the library) accordingly.