I am also having the same issue as the OP. I updated the firmware to the version you provided and it has done nothing to fix the problem. I have 5 U sticks, 4 formatted to FAT32, and the other was supplied with the printer.

Prior to the firmware update each U stick worked for a while before each stick stopped being recognised. I formatted one of the 4 U sticks, then copied the firmware to it. The printer recognised the firmware on the U stick, and then updated. The printer immediately stopped detecting any file I placed on the U stick.

Is there any other possible solution to get the printer to recognise what's on a U stick?