From both the server log, and my manual test, API for thumbnails works fine.
Please paste this url into your browser and check what you get:
https://airport-data.com/api/ac_thumb.json?m=ae07ba
It should return one record like this:
{
"status": 200,
"count": 1,
"data": [
{
"image": "https://airport-data.com/images/aircraft/thumbnails/001/200/001200583.jpg",
"link": "https://airport-data.com/aircraft/photo/001200583.html",
"photographer": "Jim Hellinger"
}
]
}