Internal server error all over my site

I need urgent help - whenever one tries to access any site of my website (except for the starting page). I get "internal sever error". I already did define('WP_MEMORY_LIMIT','256M'); in the wp-config.php.

Maybe interesting as to the history of this error: I updated seven plugins today (including WPMUDEV-Dashboard and WP_Smush) and afterwards I had the internal server error even on login-page. After moving the plugin "WPS-Hide-Login" to the "plugins2-folder" I at least could login once again. However, for the frontend of my site it did not help.

Support access is granted, however you might need an extra Login and Password to access, so as soon as anyone of you will answer I will send it via the contact form.