I downloaded the current Opencart <https://www.opencart.com/index.php?rout ... load_id=55>
When I attempted an upgrade it stopped with an error (started with json). When I read the error message I noticed that affiliate was misspelled.
The upgrade stopped due to an extra 'e' in the word affiliate (affiliatee) in this file 1009.php. I did a find and replace in the file online, and was able to complete the upgrade.