indexing – How to test multiple Sitemap.xml files?

Is there a way to test multiple sitemap.xml files?
The validation works well, Google accepts all subfiles, but "Verification of the server response" in Yandex returns "The document does not contain any text".

By the rate of analysis and the overall progress of the indexing, I have the impression that both search engines fail to read the contents of the sitemap files. Because there is a large amount of "Discovered – Currently Unindexed" = 2/3 of all content, they have never been explored and due to the low indexation of ration in Yandex.

This website contains about 750,000 links in site map files. When I generate 50,000 links per file (about 11 MB), the crawl graph goes up, then falls. When there are 10,000 links per file, the graph drops much faster and stays at about the same level.

We have done various checks and, technically, everything seems to be going well, but if you look at the performance, it's pretty dubious.
Robots.txt gives full access. robots meta tags too.

  • Can anyone suggest a way to check why "Checking the server response" returns an error when the file exists?
  • Is there a way to check if the whole sitemap file system really works – which really means being read correctly by the search engines?
  • Can this problem be related to the settings defined in the .htaccess file?

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

Thanks in advance
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