'Unknown error' caused by redirect in htaccess
It might be down to the order you have the code in your htaccess file? Did you try putting your code above the Craft rewrite code?

# redirect PHP or HTM to no extension
RewriteCond %{THE_REQUEST} ^[A-Z]{3,9}\ /([^\ ]+)\.(php|htm?)
RewriteRule ^ /%1 [L,R=301]

# Send would-be 404 requests to Craft
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_URI} !^/(favicon\.ico|apple-touch-icon.*\.png)$ [NC]
RewriteRule (.+) index.php?p=$1 [QSA,L]


Also I tend to exclude certain directories when I don't want rewrites to apply. One way to do this would be;

# ------------------------------------------------------------------------------
# | Exclude directories from the rewrite rules                                 |
# ------------------------------------------------------------------------------
RewriteRule ^craft - [L]