We did switch from Apache to LiteSpeed, that may have something to do with it.
When renaming your .htaccess the site works, so it would appear that there's something in that causing the problem. I see a note at the top of it that it was edited today, so whatever you did edit may be the problem.
I'd say it relates to this in your error log: MIME type [application/x-httpd-php5.6] for suffix '.php' does not allow serving as static file, access denied!
I also saw in the .htaccess that you have a path for autoprepend that is incorrect (it changed when we moved to DA), and should be in the form: /home/USER/domains/DOMAIN/public_html