403 error when clicking on "More..." link in the block

- Do you have clean urls on or off? Have you tried it without?

Yes, my clear urls are on. Amazingly, after I read your reply I tried to switch it off and, bingo, it worked. But I need clean url!

- Did you make adjustments to your .htaccess? If so, which?

I think some modules like Gallery.module have made their own adjasments to .htaccess. Currently, my .htaccess file looks like:

# BEGIN Url Rewrite section
# (Automatically generated.  Do not edit this section)
<IfModule mod_rewrite.c>
    RewriteEngine On

    RewriteBase /

    RewriteCond %{REQUEST_FILENAME} -f [OR]
    RewriteCond %{REQUEST_FILENAME} -d [OR]
    RewriteCond %{REQUEST_FILENAME} gallery\_remote2\.php
    RewriteCond %{REQUEST_FILENAME} !/index\.php$
    RewriteRule .   -   [L]


    RewriteCond %{THE_REQUEST} \ /gallery/photo/([0-9]+)/comment/view(\?.|\ .)
    RewriteCond %{REQUEST_FILENAME} !/index\.php$
    RewriteRule .   /index.php?q=gallery&g2_view=comment.ShowAllComments&g2_itemId=%1   [QSA,L]

    RewriteCond %{THE_REQUEST} \ /gallery/photo/([0-9]+)/comment/add(\?.|\ .)
    RewriteCond %{REQUEST_FILENAME} !/index\.php$
    RewriteRule .   /index.php?q=gallery&g2_view=comment.AddComment&g2_itemId=%1   [QSA,L]

    RewriteCond %{THE_REQUEST} \ /gallery/photo/([^?]+)(\?.|\ .)
    RewriteCond %{REQUEST_FILENAME} !/index\.php$
    RewriteRule .   /index.php?q=gallery&g2_controller=permalinks.Redirect&g2_filename=%1   [QSA,L]

    RewriteCond %{THE_REQUEST} \ /gallery/([^?]+)(\?.|\ .)
    RewriteCond %{REQUEST_FILENAME} !/index\.php$
    RewriteRule .   /index.php?q=gallery&g2_view=core.ShowItem&g2_path=%1   [QSA,L]

    RewriteCond %{THE_REQUEST} \ /([^?]+)/slideshow(\?.|\ .)
    RewriteCond %{REQUEST_FILENAME} !/index\.php$
    RewriteRule .   /index.php?q=gallery&g2_view=slideshow.Slideshow&g2_path=%1   [QSA,L]

</IfModule>

# END Url Rewrite section

#
# Apache/PHP/Drupal settings:
#

# Protect files and directories from prying eyes.
<FilesMatch "(\.(engine|inc|install|module|sh|.*sql|theme|tpl(\.php)?|xtmpl)|code-style\.pl|Entries.*|Repository|Root)$">
  Order deny,allow
  Deny from all
</FilesMatch>

# Set some options.
Options -Indexes
Options +FollowSymLinks

# Customized error messages.
ErrorDocument 404 /index.php

# Set the default handler.
DirectoryIndex index.php

# Override PHP settings. More in sites/default/settings.php
# but the following cannot be changed at runtime.

# PHP 4, Apache 1
<IfModule mod_php4.c>
  php_value magic_quotes_gpc                0
  php_value register_globals                0
  php_value session.auto_start              0
</IfModule>

# PHP 4, Apache 2
<IfModule sapi_apache2.c>
  php_value magic_quotes_gpc                0
  php_value register_globals                0
  php_value session.auto_start              0
</IfModule>

# PHP 5, Apache 1 and 2
<IfModule mod_php5.c>
  php_value magic_quotes_gpc                0
  php_value register_globals                0
  php_value session.auto_start              0
</IfModule>

# Reduce the time dynamically generated pages are cache-able.
<IfModule mod_expires.c>
  ExpiresByType text/html A1
</IfModule>

# Various rewrite rules.
<IfModule mod_rewrite.c>
  RewriteEngine on

  # If your site can be accessed both with and without the prefix www.
  # you can use one of the following settings to force user to use only one option:
  #
  # If you want the site to be accessed WITH the www. only, adapt and uncomment the following:
RewriteCond %{HTTP_HOST} !^www\.mydomain\.tld$ [NC]
RewriteRule .* http://www.mydomain.tld/ [L,R=301]
  #
  # If you want the site to be accessed only WITHOUT the www. , adapt and uncomment the following:
  # RewriteCond %{HTTP_HOST} !^example\.com$ [NC]
  # RewriteRule .* http://example.com/ [L,R=301]


  # Modify the RewriteBase if you are using Drupal in a subdirectory and
  # the rewrite rules are not working properly.
  #RewriteBase /drupal

  # Rewrite old-style URLs of the form 'node.php?id=x'.
  #RewriteCond %{REQUEST_FILENAME} !-f
  #RewriteCond %{REQUEST_FILENAME} !-d
  #RewriteCond %{QUERY_STRING} ^id=([^&]+)$
  #RewriteRule node.php index.php?q=node/view/%1 [L]

  # Rewrite old-style URLs of the form 'module.php?mod=x'.
  #RewriteCond %{REQUEST_FILENAME} !-f
  #RewriteCond %{REQUEST_FILENAME} !-d
  #RewriteCond %{QUERY_STRING} ^mod=([^&]+)$
  #RewriteRule module.php index.php?q=%1 [L]

  # Rewrite current-style URLs of the form 'index.php?q=x'.
  RewriteCond %{REQUEST_FILENAME} !-f
  RewriteCond %{REQUEST_FILENAME} !-d
  RewriteRule ^(.*)$ index.php?q=$1 [L,QSA]
</IfModule>

# $Id: .htaccess,v 1.73 2006/04/14 09:08:26 killes Exp $

- Does this bug also occur on a clean install on the same server?

I haven't tried on clean install.

Thanks!