I have followed through this tutorial:
http://www.wpbeginner.com/wp-tutorials/how-to-fix-the-internal-server-error-in-wordpress/
Done all this:
Checking for Corrupt .htaccess File (deleted it, created a new one - no effect)
Increasing the PHP Memory Limit (I checked and it´s supposed to be 64MB - which is ok)
Deactivate all Plugins (Done - no effect except they disappear in the admin panel)
Re-upload Core Files (uploaded all but wp-content folder - no use)
Ask your Hosting Provider (send email but got no response - as this problem appeared suddenly, I not sure how can it the reason)
;( Still looking for advice!