I spent some time tracking down why it fails on some networks but works on others while browsers always work: if ipv6 is available but unreachable, kuroba never retries on ipv4.

Test at will by null routing i.4cdn.org ipv6 only, images will refuse to load but 'open in browser' pops up with a very short delay. If you have a router you control, add 2606:4700::6810/96 (i.4cdn.org) to a firewall blocklist.

This is 100% repeatable if you have working ipv6 to compare with.