My sitemap became 404 in the last days

[anjali raghav]

At a brand new website for which lately there was a 301 discussion [............. ] I have noticed that in the last days my sitemap is broken.
The GSC has found it for the last time on the 3/11 and the link is https://www.example.com/sitemap_index.xml
I am using the Rankmath SEO plugin and there everything looks ok as the sitemap is active.

I have found 2 suggestions from the Yoast support page that maybe could help also in my case, but they didn't work:
1) Go to WP Admin>Permalinks and just click save
2) Add this to the .htaccess file

[Basso]

I have often read about this issue on the internet. Were you able to solve this?