Indexing – How do I test multiple Sitemap.xml files?

Is there a way to test multiple sitemap.xml files?
The validation works flawlessly. Google accepts all subfiles, but "Server Response Checker" in Yandex returns "Document does not contain any text".

The crawl rate and overall indexing progress give me the impression that both search engines can not read content from the sitemap files. Both, because there is a large amount of "discovered – currently unindexed" = 2/3 of the entire content, they were never crawled, as well as because indexing in Yandex is low.

This website contains about 750,000 links in Sitemap files. If I generate 50,000 links per file (about 11 MB), the crawl chart goes up and then drops. If there are 10,000 links per file, the graphics sink much faster and stay at about the same level.

We've done a lot of testing and technically everything seems to be fine, but in terms of performance, it's pretty dubious.
Robots.txt provides full access. Robot meta tags too.

  • Can anyone suggest a way to check why "Server Response Check" returns an error if the file exists?
  • Is there any way to test if the entire system of sitemap files really works – that is, if it's read correctly by the search engines?
  • Can this issue be related to the settings specified in the .htaccess file?

Please see screenshots below.
Location of the sitemap file: https://www.rusinfo.eu/sitemap.xml
Yandex Server Check link: https://webmaster.yandex.ru/tools/server-response/

thank you in advance
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