My Website had problems dealing with attachments (uploading seemed to result in a HTTP Error quite often) so I dug around a little.
Developer Tools showed me that the /wp-admin/async-upload.php returns an Error 502 (at least for bigger pictures and maybe all jpeg).
I also tried manually rebuilding thumbnails, but the plugins' ajax calls (I used two) also returned 502.
I have already deactivated all plugins, didn't change anything. Only idea is that maybe some media size might cause a problem but I don't remember changing anything in that direction, it just stopped working... Though it's possible that there was a major core update in that time
List of sizes:
thumbnail: 150×150 Px
medium: 300×300 Px
medium_large: 768×0 Px
large: 1024×1024 Px
iphone: 320×9999 Px
iphone_2x: 640×9999 Px
ipad_hoch: 768×9999 Px
ipad_hoch_2x: 1536×5999 Px
ipad_quer: 1024×9999 Px
ipad_quer_2x: 2048×9999 Px
custom1: 1400×9999 Px
custom2: 1700×9999 Px
custom3: 1850×9999 Px
custom4: 2100×9999 Px
klein: 360×5999 Px
mittel: 720×5999 Px
riesig: 1139×5999 Px
riesig_2x: 2278×5999 Px
sow-carousel-default: 272×182 Px
(medium_large seems to be height 0?)