HTTP Error – uploading video. Images OK

Hi,

i looked every forum and articles about WordPress HTTP Error everywhere, but nothing helped. I’ve migrated my whole webserver to a dedicated server. I use ISPCP, everything works great after migration, but my WordPress does not allow me to upload videos. I can upload pictures, but no videos. The Flash uploader says in a red marguee: HTTP Error. The browser uploader says:

Error 500!

/wp-admin/media-upload.php
Internal Server Error!

Now I tried everything: I have the latest PHP and mySQL running, I set the complete wp-content to 0775, 0777, 0755 (with all the files and directories below), I set the .htaccess file in wp-admin, I checked everything, but the error is still there (yes, I’ve restarted apache2 several times). I deleted the wordpress completely (not the wp-content) and re-uploaded it, re-configured it. Error still there. I deactivated all the plugins. Did not help.

Now I am done. Where the hell is the problem? Can anyone think of something else?

.htaccess in wp-admin
AddType x-mapp-php5 .php

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

# 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

.htaccess in root
the same or none – does not affect the error

Thank you for your advices

EDIT: oh, it runs on last Debian Squeeze (under XenServer) with all updates and upgrades on. ISPCP 1.0.7