I investigated your case. When downloading the image, the file does not exist.
I think this is due to our script making mass requests to your server and it doesn't accept as many simultaneous connections.
You should adapt either the number of API calls or configure your server to accept more connections from our server.
"and it doesn't accept as many simultaneous connections"
it seems, I am not the onliest one, who has the same problem, and it seems, as like as this problem occurs not by changing anything on the server from the sellers but it occurs by changing your upload rules.
So maybe it would be easier (for all), you will change the upload speed and decrease the mass requests.
I think, it would be easier for you, to slow down your mass requests (as like as in past) as to explain each seller to raise their server connections or to explain your buyers why you have so much items without pictures on sale.