500 Error

Ok. this is odd, I receive an international servier error: 500 when trying to view post on my site. I can view pages, and categories perfectly, however when I try to view a single post, it returns the error.

.htacess is as follows:

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>

# END WordPress

if you view the site https://www.thetvstop.com it loads, and the category https://www.thetvstop.com/tv/cw/the-vampire-diaries it loads, however if you click a post within the above category, it returns the error.