HTTP 404 with Post name permalink

Are you using XAMPP or MAMP? There are a couple of common hiccups with those environments, taken from the WordPress Codex: Fixing Permalink Problems

Users of XAMPP (Windows): Some versions of XAMPP do not enable mod_rewrite by default (though it is compiled in Apache). To enable it — and thus enable WordPress to write the .htaccess file needed to create pretty permalinks — you must open apache/conf/httpd.conf and uncomment the line LoadModule rewrite_module modules/mod_rewrite.so (i.e., delete the hash/pound sign at the front of the line).

Users of WAMP (Windows): Some versions of WAMP (all versions?) do not enable mod_rewrite or permit following SymLinks by default. To enable the required functionality navigate to the apache/conf/httpd.conf file, open with a text editor and uncomment the line LoadModule rewrite_module modules/mod_rewrite.so (i.e., delete the hash/pound sign at the front of the line). Then further down in the same file there is a section that starts with the line "Options FollowSymlinks". Change the second line in that section from "AllowOverride none" to AllowOverride all. Save edited httpd.conf and restart all WAMP modules. Your permalinks should now work.

You might also see Permalinks without mod_rewrite if your sandbox doesn't have mod_rewrite available.

Apache

If you are using Apache there are usually two other culprits to broken permalinks: .htaccess isn't being generated (because of permissions settings) or Apache's AllowOverride directive isn't enabled.

First, if you SSH into your server, do you see a generated .htaccess file in the root? If not, WordPress might not have permissions to write that file. It's also possible the file does exist, but that WordPress cannot edit it. In either case, you can chmod that file (and create it if it doesn't exist) to 666.

Next, ensure your Apache config has the following settings:

  <Directory />
    Options FollowSymLinks
    AllowOverride All
 </Directory>

Finally, read through the Fixing Permalink Problems section of the WordPress Codex. There are several other tips and suggestions on why permalinks might not work.

found this post on another site helped many people already

I finally managed to solve the problem! The solution: I was using a custom permalink structure http://kyl.fi/%category%/%postname%/. I removed the trailing slash (i.e. the last /) and voila. However, I'm quite sure I used a permalink structure with the trailing slash before without any problems, so I'm still confused and would be interested the hear more about this issue if somebody has an explanation.

All the standard permalinks have a trailing / in there.

In my case, firstly I had to update the .htaccess file inside my website root folder:

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>
# END WordPress

WordPress does this automatically if it has write permission. Otherwise it'll complain it can't write to it, and give the above code sample so you can manually update the .htaccess.

After that, I edited the apache2.conf file. In Linux, it resides in /etc/apache2/apache2.conf, there will be a section like this:

<Directory /var/www/>
    Options Indexes FollowSymLinks
    AllowOverride None
    Require all granted
</Directory>

Change AllowOverride None to AllowOverride FileInfo.

Finally, execute the following commands:

sudo a2enmod rewrite
service apache2 restart

All these steps were necessary in order to work.