Trying to print test strips on my Pro-10 using a letter-size piece of Entrada Bright 300. The first strip printed fine, but I can't print on the paper again. Everytime I try, the paper feeds in and the then the printer says that the wrong size paper is loaded. I have cycled the power on the printer and, although the first strip printed fine when fed through the main feed tray, I also tried choosing the paper size as "Letter Art Paper (margin 30)" and feeding through the rear slot, but the problem persists.

The printer uses a sensor on the head to measure the width of the sheet when it is loaded. If there is an image already on the sheet, the printer reads that as the edge of the paper, and generates the error you're seeing. I have not tested, but it may be possible to turn this feature off- open the Canon printer utility for the Pro-10, and in the Custom Settings section uncheck the box for 'Detect paper width'. Then try loading the sheet again. If the error persists, you'll need to cut a custom sheet size for your test strips.