When URL of images have Spaces like "http://imagehost.com/album/user23242242/store image/a nice day.jpg" then the script fails to reproduce valid image.
The image is only visible inside the editor and not in the post.
I think you have to check the URLs for spaces and replace them with ...
I try to solve the problem by adding the $url = preg_replace('/ /', ' ', '$url'); in ProxyListener.class.php but is not working...
Maybe needed to apply this patch over the IMG BBcode.
This Plugin is AWESOME, and it helps my to keep my Site (SSL) clear of mixed security contents warnings.
Now all imgs are outgoing through internal source (Proxy) and the SSL is clear... 
The only negative is that adding a little more delay in Page Speed but is ok..
My forums are at https://xiaomi-miui.gr/community
BTW : Why I can't see any content of any forums ?
All the forums are empty for me, but I see that existing many posts which are not visible by me.