Hello @onurparlar
It seems like your server is running out of memory when opening the Sitemaps.
Please change this value to 200:
https://i.rankmath.com/RougQv
You can also try to increase the PHP memory limit, execution time by requesting it from the hosting guys.
Once done, please delete all the older Sitemaps in Google and add only the index Sitemap that will look like:
https://rankmath.com/sitemap_index.xml
For the final issue, assuming you are using the latest version of all the plugins and the themes (please update if not yet), it would seem like a conflict with one of the plugins or the theme you are using.
To determine that, please follow this troubleshooting guide:
https://rankmath.com/kb/check-plugin-conflicts/
Please let us know if you find the culprit.
Only the site admin would know and your users won’t be affected by this troubleshooting.
Looking forward to helping you. Thank you.