The ImgBB image isn't showing up for me at this time, and seems like it's hit and miss.
There could be few reasons for it.
1. Try doing a hard refresh Ctrl+Shift+R. This will clear your cache and reload everything.
2. Your internet service provider might flag the site, I don't see any reason for that though.
I don't think it's up to him or his provider after all. the same thing is happening to me (I'm sure we don't have the same ISP)
ImgBB most often I used it myself, I noticed that after two reloads of the page, some of the img managed to show up. I did an update of one
thread and half of the images were visible.
I would say obviously the problem is on their side.