Problems with Drupal in a subdirectory (500 Internal Server Error)

Hello everyone,
I work on the maintenance of an old website running Drupal 6. Yesterday I tried to update Drupal Core to version 6.32, and experienced this error. I'm not sure it wasn't already present:

Internal Server Error

The server encountered an internal error or misconfiguration and was unable to complete your request.

Please contact the server administrator, postmaster@istitutotolman.net and inform them of the time the error occurred, and anything you might have done that may have caused the error.

More information about this error may be available in the server error log.

This is my configuration:
My main Drupal installation (the one affected by the problem) lives in /web subdirectory of my hosting space (provided by Aruba.it) at http://istitutotolman.net/, while other subdirectories contain other CMSes and static content.

So I have:

• /
  • /cba
  • /static
  • ...
  • /web (this should be served by deafult)

In my root directory I have this .htaccess file:

RewriteEngine On #created by aruba do not touch this file!
RewriteCond %{REQUEST_URI} !^/web
RewriteCond %{REQUEST_URI} !^/drupal
RewriteCond %{REQUEST_URI} !^/cba
RewriteCond %{REQUEST_URI} !^/static
RewriteCond %{REQUEST_URI} !^/contenuti
RewriteCond %{REQUEST_URI} !^/lychee
RewriteRule ^(.*)$ web/$1 [L]

And it seem to work fine, since when I try to go to http://istitutotolman.net/ it serves me my Drupal homepage from /web. Anyway, when I try to go right to http://istitutotolman.net/web/ (or click a link that points there) it gives me that Internal Server Error.

This error doesn't appear when I try to visit another Drupal 6 installation that lives on the /cba subdirectory pointing my browser to http://istitutotolman.net/cba.

What should I do? I'm sure there's something wrong somewhere, but can't find how to fix it...

Any help or suggestion will be really appreciated,

regards
gerlos

PS For your reference, here's my .htaccess file from /web subdir (I did a couple of tweaks to make it work with my hosting service):

# Apache/PHP/Drupal settings:
#

# Protect files and directories from prying eyes.
<FilesMatch "\.(engine|inc|info|install|make|module|profile|test|po|sh|.*sql|theme|tpl(\.php)?|xtmpl|svn-base)$|^(code-style\.pl|Entries.*|Repository|Root|Tag|Template|all-wcprops|entries|format)$">
  Order allow,deny
</FilesMatch>

# Don't show directory listings for URLs which map to a directory.
#Options -Indexes

# Follow symbolic links in this directory.
#Options +FollowSymLinks

# Make Drupal handle any 404 errors.
ErrorDocument 404 /index.php

# Force simple error message for requests for non-existent favicon.ico.
<Files favicon.ico>
  # There is no end quote below, for compatibility with Apache 1.3.
  ErrorDocument 404 "The requested file favicon.ico was not found.
</Files>

# 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
  php_value mbstring.http_input             pass
  php_value mbstring.http_output            pass
  php_value mbstring.encoding_translation   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
  php_value mbstring.http_input             pass
  php_value mbstring.http_output            pass
  php_value mbstring.encoding_translation   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
  php_value mbstring.http_input             pass
  php_value mbstring.http_output            pass
  php_value mbstring.encoding_translation   0
</IfModule>

# Requires mod_expires to be enabled.
<IfModule mod_expires.c>
  # Enable expirations.
  ExpiresActive On

  # Cache all files for 2 weeks after access (A).
  ExpiresDefault A1209600

  <FilesMatch \.php$>
    # Do not allow PHP scripts to be cached unless they explicitly send cache
    # headers themselves. Otherwise all scripts would have to overwrite the
    # headers set by mod_expires if they want another caching behavior. This may
    # fail if an error occurs early in the bootstrap process, and it may cause
    # problems if a non-Drupal PHP file is installed in a subdirectory.
    ExpiresActive Off
  </FilesMatch>
</IfModule>

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

  # If your site can be accessed both with and without the 'www.' prefix, you
  # can use one of the following settings to redirect users to your preferred
  # URL, either WITH or WITHOUT the 'www.' prefix. Choose ONLY one option:
  #
  # To redirect all users to access the site WITH the 'www.' prefix,
  # (http://example.com/... will be redirected to http://www.example.com/...)
  # adapt and uncomment the following:
  # RewriteCond %{HTTP_HOST} ^example\.com$ [NC]
  # RewriteRule ^(.*)$ http://www.example.com/$1 [L,R=301]
  #
  # To redirect all users to access the site WITHOUT the 'www.' prefix,
  # (http://www.example.com/... will be redirected to http://example.com/...)
  # uncomment and adapt the following:
  # RewriteCond %{HTTP_HOST} ^www\.example\.com$ [NC]
  # RewriteRule ^(.*)$ http://example.com/$1 [L,R=301]

  # Modify the RewriteBase if you are using Drupal in a subdirectory or in a
  # VirtualDocumentRoot and the rewrite rules are not working properly.
  # For example if your site is at http://example.com/drupal uncomment and
  # modify the following line:
  # RewriteBase /drupal
  #
  # If your site is running in a VirtualDocumentRoot at http://example.com/,
  # uncomment the following line:
  RewriteBase /web

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

# $Id$