It also happened to me, but I wasn't using imgur as my image hosting site I used imgbb.com. Although, it didn't display any error like that of imgur but still yet the forum could render the image, so I had to delete the whole post because I was quoting an Op for visibility.
This could happen for various reasons... For example: was the picture larger than 2.5 MB? If yes, this is the reason.
Another reason: how did you try to upload the picture on the forum? Simply by adding its link between img-/img brackets? I mean this link: https://imgur.com/a/i5LE0HB.
If yes, again, this is the reason. You don't have to put between img-/img brackets the direct link to the pic. Instead, you have to have to click on the uploaded picture, in order to open it in a new tab, then right-click on the pic after you uploaded it on ImgBB and select "Copy link" or "Copy image location". By doing so, you'll get a link ending in ".png" or ".jpg". And that is the link you should insert between img-/img.
Example: if you'll type this inside your post, no image will be displayed:
Code:
[img]https://ibb.co/25JPKx9[/img]
Trying to write the above BBCode sequence inside your topic will end up in this:
That is a picture I uploaded on ImgBB. Now I open it in a new tab. For doing that I either click on it or copy that URL and paste it in a new tab. Once the image is displayed in a new tab I right-click on it and select "Copy image location". This generates a new URL, ending in ".png": https://i.ibb.co/7bhVyPf/Untitled4.png. And this is what you have to place between img-/img:
Code:
[img]https://i.ibb.co/7bhVyPf/Untitled4.png[/img]
Now the image is displayed:
