.htaccess give 404 Not Found when url in SEO-friendly URL

Question

Im using opencart fresh install and using theme from here. After i enable SEO setting in admin and rename .htaccess file then set SEO keyword i can't open my product page. It show 404 error not found.

This website still under developement you can check here http://203.223.152.159/~happymas/

Product with SEO -> http://203.223.152.159/~happymas/this_is_seo

.htaccess 

# 1.To use URL Alias you need to be running apache with mod_rewrite enabled. 

# 2. In your opencart directory rename htaccess.txt to .htaccess.

# For any support issues please visit: http://www.opencart.com

Options +FollowSymlinks

# Prevent Directoy listing 
Options -Indexes

# Prevent Direct Access to files
<FilesMatch "\.(tpl|ini|log)">
 Order deny,allow
 Deny from all
</FilesMatch>

# SEO URL Settings
RewriteEngine On
# If your opencart installation does not run on the main web folder make sure you folder it does run in ie. / becomes /shop/ 

RewriteBase /
RewriteRule ^sitemap.xml$ index.php?route=feed/google_sitemap [L]
RewriteRule ^googlebase.xml$ index.php?route=feed/google_base [L]
RewriteRule ^download/(.*) /index.php?route=error/not_found [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_URI} !.*\.(ico|gif|jpg|jpeg|png|js|css)
RewriteRule ^([^?]*) index.php?_route_=$1 [L,QSA]

### Additional Settings that may need to be enabled for some servers 
### Uncomment the commands by removing the # sign in front of it.
### If you get an "Internal Server Error 500" after enabling any of the following settings, restore the # as this means your host doesn't allow that.

# 1. If your cart only allows you to add one item at a time, it is possible register_globals is on. This may work to disable it:
# php_flag register_globals off

# 2. If your cart has magic quotes enabled, This may work to disable it:
# php_flag magic_quotes_gpc Off

# 3. Set max upload file size. Most hosts will limit this and not allow it to be overridden but you can try
# php_value upload_max_filesize 999M

# 4. set max post size. uncomment this line if you have a lot of product options or are getting errors where forms are not saving all fields
# php_value post_max_size 999M

# 5. set max time script can take. uncomment this line if you have a lot of product options or are getting errors where forms are not saving all fields
# php_value max_execution_time 200

# 6. set max time for input to be recieved. Uncomment this line if you have a lot of product options or are getting errors where forms are not saving all fields
# php_value max_input_time 200

# 7. disable open_basedir limitations
# php_admin_value open_basedir none

Answer 1

If your htaccess file is located here: http://203.223.152.159/~happymas/, and you have your rewrite base set to /, then it's trying to rewrite to http://203.223.152.159/index.php which doesn't exist (results in a 404).

You need to set your rewrite base to /~happymas/ until you move it to a /:

RewriteBase /~happymas/