The problem is probably from imgur. Their site now automatically redirects from the old domain imgur.com to imgur.io
Not sure why the did this but yes indeed the images with the imgur.com link are not showing in the forum.
are you perhaps using a smartphone? I searched imgur.com using a PC and it still brings me to imgur.com but when I use my smartphone and searched Imgur.com it brings me to imgur.io. also, if I use the URL from the BBCODE provided by Imgur on the address bar it still brings me to the image that was uploaded on their website.