A red cross in the charging icon indicates that it is detecting that it is connected to a non-compatible charging source.
It determines the source by measuring the float voltage on the data pins.
Are you using the original cable that came with the charger? some 'charger' cables don't supply data pins or they break in the cable, causing your legit charger to look like a USB port.
Try a different cable, or try charging with your tablet switched off (bypasses the charger test)