[SOLVED] “HTTP error” when uploading

As noted in https://wordpress.org/support/topic/media-library-is-slow-lagging-constantly-spinning-or-crashes/, if you’re seeing a “HTTP error” message in the Media Library (especially after trying to upload a SVG image), then it might be a problem between SVG Support and php7.

The fix, as described in that post, seems to be to install php7.0-xml (with apt-get install php7.0-xml) then restarting your web server.