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Not only imgur.com, but I remember that I got an error using another hosting service, I don’t remember it specifically, but I did not face any problem with TalkImg.com, so even if you have an image hosted in hosting services, it is better to re-upload it using TalkImg.com.
TalkImg.com and imgbb.com is the two image hosting sites that work well for forum. I'm fine with both sites since imgur has been problematic - but to avoid the same problems in the future, TalkImg.com tends to be the best recommendation at the moment. I don't re-upload all old image - except for a few that I think are important.
TryNinja has created a script to automatically fix invalid images - so this is very useful than doing it manually. I haven't done it yet - but might try it in the future if the script still works.