Based on this it appears that the external site is rate limiting requests from a specific Discourse forum?
I just started having problems with a 429 error for Imgur oneboxed images on my Discourse instance. But the same links work here:
https://i.imgur.com/6IXPzY6.png
Strangely, without the .png part of the link the onebox does appear:
https://i.imgur.com/6IXPzY6
I tried adding https://i.imgur.com and https://imgur.com to force custom user agent hosts , but it doesn’t make any difference.