Welcome to the HP Forums. I would be happy to help you with your HP Officejet Pro 8500A printer. I understand that you are getting paper mismatch error message and you have already done some preliminary troubleshooting.
Don't worry though as I have a few steps to determine the issue and find a solution as well,
Reset the printer.
Adjust the paper size setting.
In the program you want to print from, click File, and then click Print.
Select your printer from the Printer drop-down list.
Click Page Setup, and then click Paper Size.
Select the paper size that matches the size and type of paper loaded in the printer tray, and then click OK. If there are no matching paper sizes, select the size from the list that is closest to the size of the paper loaded in the tray.
Try to print.
Click here and refer step 1 in the document to perform above troubleshooting steps.