One of my sites started having this problem last year. A few posts had tips that didn't work for me and then I forgot about the broken site. I finally prodded the hosting company and they downgraded my PHP to 5.2 and the problem went away. I tried editing the HTMACCESS file, adding a PHP.ini file, but none of the more complex suggestions. There haven't been many posts on this since 2012. I'm running a very recent version of C5.