Hi, I try to keep my plugin compatible with Polylang and I'm not aware of any problems that cause an error 500.
@emotionaltrash are you using Polylang? Does the sitemap index (main sitemap.xml) load correctly? If you've got a large number of posts or are on a server with not too much memory available for PHP processes, you might want to try to set "Split by: Month" under the Sitemap options on Settings > Reading.