Upload HTTP Error

When you get the "dreaded" HTTP Error during an image upload, there are several things to look at:

Disable EWWW IO, do you still get the error? If so, then it's obviously not from EWWW IO and you can look elsewhere!
What sort of images are you uploading? Are they large? Are they PNG images? Always send some examples of images you have trouble with, when you request our help. If your images are very large, or at least large for a PNG image (over 300kb), they can be very slow to optimize. Also look at the EWWW IO Resize tab to see how many thumbnails are registered on your site. If you have 30, and background mode isn't working (see #3), that's a huge issue. If you have 5 thumbnails, you can at least rule that out.
If it happens right after an upgrade, it's possible that background/async mode had not yet re-enabled itself. After every upgrade, the background mode is disabled and re-tested. Once the test succeeds, then background mode is re-enabled automatically. If the optimization is taking too long and background mode was disabled, then it can timeout. Check the settings page, and see if background mode is working. If it is, then retry your upload and see what happens.
Does the upload throw an error after a few seconds, or does it take at least 30 seconds? Do an upload and actually time it, don't just guess, that's not very helpful :) If it fails within a few seconds, you likely need to increase your PHP memory_limit setting (check with your webhost if you don't know how). If it fails after 30+ seconds, try increasing the max_execution_time setting in PHP.
Enable the WordPress debug mode: https://codex.wordpress.org/WP_DEBUG (do this before 5 & 6).
See if the browser console gives any clues, see the video below. Note, that it's possible to do this in Chrome also by going to the Network tab, and filtering to view only XHR requests.
Check the server error logs. If you don't know what those are, where those are, or just can't find them, ask your webhost to check them. You'll need to upload an image, and mark down the exact time when the upload failed so that you know when to look in the logs (your webhost will also need to know that if you ask them to look).

If you still need to contact us, be sure to include details you found from all of the steps you tried.