The Concept: I wish to allow it to be that something that uses the domain title and finishes with .html is treated like a get variable by index.php

Example: www.test.ro/1/2/3.html should really be www.test.ro/index.php?var=1/2/3.html.

www.test.ro is setup like a virtual host for development, and AllowOverride has value All.

The .htaccess file appears to become processed, although not constantly. Basically write a non-recognized rule like 'BizzareRule', the server works without coming back a code 500 error. Basically put correct rules between <IfModule mod_rewrite.c></IfModule>, I recieve an 500 error, despite the fact that I've other vhosts utilizing the same conditions and dealing perfectly.

Listed here are the items in my vhost:

<VirtualHost *:80>
    ServerAdmin eu@localhost.com
    ServerName  www.test.ro
    ServerAlias test.ro

    DocumentRoot D:/Projects/grabsite/test.ro
    Options Indexes FollowSymLinks MultiViews

    DirectoryIndex index.php index.html

    LogLevel warn
    ErrorLog D:/Projects/grabsite/test.ro/error.log
    CustomLog D:/Projects/grabsite/test.ro/access.log combined

    <Directory "D:/Projects/grabsite/test.ro">
        Options -Indexes FollowSymLinks
        AllowOverride All
        Order allow,deny
        Allow from all
    </Directory>
</VirtualHost>

And test content for .htaccess:

RewriteEngine On
RewriteRule ^/?([^/]*)\.html$ /index.php?seo=$1 [L]

It appears that you simply makes its way into in infinite loop. Try first to simplify the rule: RewriteRule ^(.*)\.html$ /index.php?seo=$1 [L]

Or simply RewriteRule ^index.html$ /index.php?seo=$1 [L]

Next, it is best to have RewriteCond -f and RewriteCond -d so as not to complete the rule if you're asking for existing file/folder. In certain situations, this could prevent an infinite loop within the rule

The issue is in regex pattern. You stated: "The .htaccess file appears to become processed, although not constantlyInch.

Should you look at your pattern^/?([^/]*)\.html$ you will find that this works for files in root folder ONLY (e.g. /index.html).

Solution: change pattern to ^(.+)\.html$ and all sorts of is going to be fine -- now it'll match ANY URL that finishes with .html (e.g. /index.html in addition to /hello/pink-kitten.html etc).