domain.com/abc works but domain.com/abc/ gives Error 500

Hey Guys.
I am working on optimizing a website and trying to create a better link structure for my client.
However I have a problem

http://www.aieus.com/faq works perfect.
http://www.aieus.com/faq/ gives me 500 internal server error.

here is my .htaccess code. please inform me if i did something wrong or missing something

Options +FollowSymlinks
RewriteEngine on
RewriteBase /
rewritecond %{http_host} ^aieus\.com [nc]
rewriterule ^(.*)$ http://www.aieus.com/$1 [r=301,nc]


#Fix Rewrite
Options -Multiviews


## EXTENSION REWRITE HIDE PHP#
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME}\.php -f
RewriteRule ^(.*)$ $1.php [L,QSA]


## Errors ##
ErrorDocument 404 /404.php

 

Look inside your settings, it might have to do with canonical URLs or something

 

Adding this should fix it.

RewriteRule ^(.*)/(.*)$ $1.php [L,QSA]

Code (markup):

Tbh I don't really know how it works with RewriteConditions. Guess you've got to find that out by yourself

 

Hey, this has given me the visa versa effect.

d.com/abc/ worked fine and I got 404 for d.com/abc

have any other idea?

 

RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME}\.php -f
RewriteRule ^(.*[^/])$ $1.php [L,QSA]
RewriteRule ^(.*)/(.*)$ $1.php [L,QSA]

Code (markup):

Works for me.

 

And if none of that helps, post the error log entry here.