Is it possible you have two paged archive addons within the addons folder? Maybe one is slightly renamed? If this is true, delete all copies but one.
You can also re-download the Pixelpost package and grab a fresh paged archive addon from there and re-upload it to your server to rule out any corrupt file issues.
Also, can you please post up your manual code changes. If you did it right, there should be no question that the maximum # of thumbnails should change accordingly.