Hi,
I am also getting same Internal Server Error 500. It occurs at 10:00 at each morning. Some process is updating htaccess file and add </FilesMatch> tag.
Thu Oct 09 10:11:39 2014] <FilesMatch> was not closed., referer:
Could please let me know how did you fixed this issue ?, even WP_MEMORY_LIMIT is 256M.
Thanks