Staging site down

The admins of a staging wordpress site wrote that the site was down, while another wordpress website on the same physical server was up. Apparently it was an "Internal Error" - so, I directed them to

https://www.wpbeginner.com/wp-tutorials/how-to-fix-the-internal-server-error-in-wordpress/

and later, passed on some more information - there seems to be some plugin / redirect which is related to the robots.txt - 

https://staging.ourdomain.org/robots.txt

works fine, and also the link

https://staging.ourdomain.org/sitemap.xml

(which actually does not exist, which is being generated. But the sitemap only links to other xml files, which leads to the 500 error.)

The current settings of the site prevent any other page from loading.