A few days ago, images from a website I maintain stopped being displayed in Gmail. The Google's Image Cache Proxy returns 404 on every image. I can confirm that they are accessible on the server itself and the newsletter displays fine outside gmail.com, even in Gmail's own mobile app (it apparently does not use Google Image Proxy). I can even confirm, that Google's bot fetches these images regularly from the server itself with the server responding 200 status and sending the image. The images in question are png and jpg, no svgs (which appear to have had problems in the past according to my research)
Is there a known remedy for this and why is Google this evil? Is there a special header that should be set for this? Like Cache-Control header that is required by their Image Proxy specifically?