HTTP Error when uploading mp4 video file

In my case the cause was plugin All In One WP Security an its “Basic Firewall Settings”. The above firewall features are applied via your .htaccess file and one if these features is “Limit file upload size (10MB).”

So to make upload of files larger then 10 MB working again you can do following:

1) Go to WP Security / Firewall / Basic firewall rules tab and uncheck “Enable Basic Firewall Protection”. Then Save settings. This will disable all Basic firewall settings and this 10 MB upload limitation as well.

or

2) If you want to keep the basic firewall rules but remove only that limitation, you need to use “Custom Rules”. Go to WP Security / Firewall / Custom rules tab and check “Enable Custom .htaccess Rules:”. Then put in the custom rules text area:

LimitRequestBody 64000000

That will allow approx. 64 MB uploads. Also make sure that “Place custom rules at the top” checkbox is NOT checked, so it overwrites the 10M limit from above.