Thank you! Found it:
Under the Widgetlogic we wrote:
is_single(‘site1’); is_single(‘site2’)
The ; in this line caused the error.
That is the error code:
mod_fcgid: stderr: PHP Parse error: syntax error, unexpected ';' in /var/www/vhosts/domain.de/httpdocs/wp-content/plugins/widget-logic/widget_logic.php(284) : eval()'d code on line 1
Maybe it is somewhere else?