Found this closed thread https://www.tvmaze.com/threads/2280/gateway-time-out-on-bulk-upload
It's still happening.
Uploading to https://www.tvmaze.com/shows/15296/voltron-legendary-defender, got this error twice. Images are uploaded though, however it took about 10 times long as it usually happens, and it looks like it's still downloading images after an error was displayed. Uploading 6 images to other Netflix show took about 5 seconds, here it took about 50 seconds for 7 images (and only 4 of them were uploaded at the moment).
Maybe it's somehow related to the fact that Voltron is country-locked on Netflix, not global?