Hi all,

my hosting provider Lanten has sent me the detailed description of the solution. They did a real great job by investigating this problem. Thank you guys!

Here do I share a translated version with some minimal modifications:

1. Usually it happens, that PHP runs as user and group 'www-data', altough the admin of the domain is another user and group.
- In this case, if admin creates a folder, PHP will only be able to write it if the folder's attributes are set to xx7.
- When PHP creates a file or a folder, administrator will not be able to access it.

These cases can be solved by using ACL-s, as we have applied it for your domain.

2. Other critical thing is the safe_mode option. For security reasons _sometimes_ this mode does not allow folders to be accessed, that have 777 attributes. So we had to disable this for your domain.

3. The third problem was caused by the .htaccess file. Our webservers have very strict security settings, so only a few commands are allowed in .htaccess files. (eg. RewriteRule or ErrorDocument) All .htaccess errors cause 500 error. So if there are any prohibited commands or any syntactical mistakes in your .htaccess files, you will receive this reply (500 error).

These three problems can occur at any other hosting providers and can cause drupal installations working not perfectly.

Generally, Content Management Systems like Drupal (or else) do not really match to the security needs of hosting providers, becouse they need a lots of manual tweeks in server side config files.