I am attempting to upload the following image "from the web" into a post:
When I do this, the dialog states "Failed to upload image; the format is not supported".
I have been able to narrow it down, thanks to Mhmd's comment below, to URLs that contain (at least) spaces (I tested on my own server with a file named "astro%20naut.jpg", after URL encoding, although I cannot share the URL).
If I upload direct to imgur from such a URL, there is no problem. It appears to be some issue on the SE side of things.