Imgur have made the decision to block the UK following being threatened by the UK IPO due to the Online Safety Act.
Unfortunately this Lemmy server resides in the UK and is not unable to access imgur as a result. The image failures you may be expieriencing is due to this instance fetching images on your behalf to protect your internet identity (image_mode: ProxyAllImages)
I am working on a work-around which is almost ready to deploy. I expect images to stop functioning completely for a short while whilst I migrate pict-rs to another system (thank god I used S3 for pict-rs storage, else this migration would take days).
Update: pict-rs wasn't migrated, instead its outbound traffic has been routed out of another region. Unfortunately attempting to purge the cache of imgur images so it'll redownload is too much of a headache. The cache is set to 7-days but it seems setting it to 1-hour doesn't affect existing images, only new ones.
If you are in the UK, then new posts that use imgur.com images shall be viewable. However this will depend on your client. For example Boost for Android gets the image URL direct from the original server, so I see the the thumbnail OK (cached via lazysoci.al) and then the "imgur has blocked you" full image (due to Boost going direct).
I expect this will only get worse.