error "An internal server error occurred. Please try again later." when trying to install or update a plugin as well as reinstalling Wordpress from the Backend.
Seems like a non-apache error message, as nothing is logged in apache's error logs.
This error survives a re-setup of wordpress and occurs again if Tools -> Import Data is made from a previously exported XML Backup.
htaccess was cleaned of all caching stuff the previous WP installation has written inside. Also deflate is turned off even if that shouldn't be a problem of "no caching plugins installed"
WP's maintenance mode which should stay activated if a plugin installation is suddenly aborted didn't stay active...