mod_rewrite Internal Server Errors

I'm working locally on a Perch website using MAMP. I have been able to get mod_rewrite working to hide extensions with no problem, but I can't blog pages to work. I get Internal Server Error messages. I have the same issue when uploading to my MediaTemple server.

This is my .htaccess file:

<IfModule mod_rewrite.c>

Options +FollowSymLinks
RewriteEngine On

# RewriteCond %{HTTP_HOST} ^blackhawkpetcare.com.au [NC]
# RewriteRule ^(.*)$ https://www.blackhawkpetcare.com.au/$1 [L,R=301]

# Redirect to PHP if it exists.
# e.g. example.com/foo will display the contents of example.com/foo.php
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME}.php -f
RewriteRule ^(.+)$ $1.php [L,QSA]

RewriteRule ^post/([a-zA-Z0-9-/]+)$ /post.php?s=$1 [L]

</IfModule>

Redirect 301 /about /about-blackhawk

Any ideas on what might be wrong?