Anyone searching for this in the future -- please just refer to http://b2evolution.net/man/installation-upgrade/advanced-setup/multiple-domains

Especially important to make sure your domain alias does not result in a redirect (before going any further, make sure it is functioning as a bare alias).

I finally fixed that by using the .htaccess file - but now I have no content anymore!

www.miq.es
www.marcoplast.com
http://made-in-quality.de/
http://computer-doctor.marbella2000.de/

# Apache configuration for b2evolution's base folder
	# Lines starting with # are comments.
	# Updated 2013-07-16
	 
	# PHP / SECURITY:
	<IfModule mod_php5.c>
	  # On some overly "secured" systems, the following may need to be added in each folder:
	  # AddHandler application/x-httpd-php5 .php
	  
	  # The following will try to configure PHP in the way b2evolution would like it to be:
	  php_flag register_globals Off
	  php_flag display_errors On
	  php_value error_reporting E_ALL
	  
	  # You may also need some of the following:
	  # Increase memory limit
	  # php_value memory_limit 256M
	</IfModule>
	 
	# DISABLE LISTING FILES ON SERVER:
	# Don't show directory contents (not desirable, especially for media folders)
	Options -Indexes
	# Note: if you have access to the command line of your server, it would probably
	# be even better to type this:  a2dismod autoindex
	# (This disables the directory listing module completely)
	 
	# DEFAULT DOCUMENT TO DISPLAY:
	# In case no specific file is requested, we want to fall back to index.php
	# In case some subdirectories are accessed, we want to display index.html
	<IfModule mod_dir.c>
	  DirectoryIndex index.php index.html
	</IfModule>
	 
	# CLEAN URLS:
	# The following will ensure you can pass extra params after the filename on the URL
	AcceptPathInfo  On
	 
	# CATCH EVERYTHING INTO B2EVO: (even if no php file appears in the URL)
	# The following will allow you to have a blog running right off the site root,
	# using index.php as a stub but not showing it in the URLs.
	# This will add support for URLs like:  http://example.com/2006/08/29/post-title
	<IfModule mod_rewrite.c>
	  RewriteEngine On
	 
	  # This line may be needed or not.
	  # enabling this would prevent running in a subdir like /blog/index.php
	  # RewriteBase /
	 
	  # Redirect anything that's not an existing directory or existing file to index.php:
	  RewriteCond %{REQUEST_FILENAME} !-d
	  RewriteCond %{REQUEST_FILENAME} !-f
	  RewriteRule . index.php [L]
	</IfModule>

You just need to change the security policy of $allow_redirects_to_different_domain in the [[advanced-php]] config file.