Double sided printing - paper error

When I try to print double sided to an AirPrint Epson printer, the last page is printed and then there is a printer error saying that the incorrect paper is loaded. I have never had this error before and printing double sided from my iMac works fine as usual. The correct paper is loaded and double sided is selected in the iPad print app. The same thing happens with my iPhone. Any suggestions as to how to cure this problem?