news-sitemap.xsl not found (500 error)

Hi there, we bought the Yoast News SEO for WordPress plugin. But we see a white page on /news-sitemap.xml because the /news-sitemap.xsl can’t be found. When visiting that .xsl-file we recieve a 500-error.

Now we found a guide on Yoast but that’s not fixing the issue. What we did:
– It’s not in the .htaccess as stated on the yoast supportpage.
– It’s not in the max.entries per page as stated here (tried 50 and 500).
– We de-activated and activated the plugin to flush the cache.
– All plugins de-activated and switched to an unaltered theme. The issue remains.
– We saved the permalinks. Without any result.

So we don’t have any other clue to get this solved. Does anybody have an idea?