When I edit a page/block and include widgets / some unspecified/unknown combination of 'things', I get a 404 error when I click save.
It seems to occur whenever HTML code is dropped into the content box, or if I use widgets etc. At the moment I am getting around this by dropping my content into phpmyadmin and saving it that way.
I'm using the .htaccess file that came with Magento, and running Magento CE 2.1.2.