I read a lot of questions related to this problem, but it seems nothing applies to this case. From the Google Search Console, the sitemap for a Wordpress website using Yoast is reported as "couldn't fetch". No other info seems to be provided.
I found similar questions everywhere, like Google Search Console Sitemap Can't Fetch.
Typical answer is: "just wait". I have been waiting for many months now. Nothing changed. Tried to re-submit many times, but nothing changed again.
I also tried "URL inspection": "No: 'noindex' detected in 'X-Robots-Tag' http header" is returned, which I understand is correct for a sitemap, and "Page fetch" is reported as successful. So Google can fetch from there, but not from the sitemap tag.
I also tried to create minimal XML to debug the issue, but "couldn't fetch" remains in any case. I tried other online services to check the URL, and every service reports the sitemap as valid. Bing has no problems with the same sitemap and indexed the site properly.
Any other idea that could lead to understand the problem? Thanks.
EDIT: This is an example of GET call with metadata for the sitemap index:
* Trying 184.108.40.206:443... * Connected to ... (...) port 443 (#0) * ALPN: offers h2 * ALPN: offers http/1.1 * CAfile: /etc/ssl/certs/ca-certificates.crt * CApath: none * TLSv1.3 (OUT), TLS handshake, Client hello (1): * TLSv1.3 (IN), TLS handshake, Server hello (2): * TLSv1.3 (IN), TLS handshake, Encrypted Extensions (8): * TLSv1.3 (IN), TLS handshake, Certificate (11): * TLSv1.3 (IN), TLS handshake, CERT verify (15): * TLSv1.3 (IN), TLS handshake, Finished (20): * TLSv1.3 (OUT), TLS change cipher, Change cipher spec (1): * TLSv1.3 (OUT), TLS handshake, Finished (20): * SSL connection using TLSv1.3 / TLS_AES_256_GCM_SHA384 * ALPN: server accepted http/1.1 * Server certificate: * subject: CN=... * start date: Jun 22 08:17:08 2022 GMT * expire date: Sep 20 08:17:07 2022 GMT * subjectAltName: host "..." matched cert's "..." * issuer: C=US; O=Let's Encrypt; CN=R3 * SSL certificate verify ok. > GET /sitemap_index.xml HTTP/1.1 > Host: ... > User-Agent: curl/7.84.0 > Accept: */* > * TLSv1.3 (IN), TLS handshake, Newsession Ticket (4): * TLSv1.3 (IN), TLS handshake, Newsession Ticket (4): * old SSL session ID is stale, removing * Mark bundle as not supporting multiuse < HTTP/1.1 200 OK < Server: nginx/1.23.0 < Date: Thu, 21 Jul 2022 09:29:33 GMT < Content-Type: text/xml; charset=UTF-8 < Content-Length: 822 < Connection: keep-alive < X-Powered-By: PHP/7.4.30 < X-Robots-Tag: noindex, follow < Vary: Accept-Encoding < Strict-Transport-Security: max-age=31536000; includeSubDomains; preload < <?xml version="1.0" encoding="UTF-8"?><?xml-stylesheet type="text/xsl" href="//.../wp-content/plugins/wordpress-seo/css/main-sitemap.xsl"?> <sitemapindex xmlns="http://www.sitemaps.org/schemas/sitemap/0.9"> <sitemap> <loc>https://.../post-sitemap.xml</loc> <lastmod>2022-04-10T09:44:18+00:00</lastmod> </sitemap> <sitemap> <loc>https://.../page-sitemap.xml</loc> <lastmod>2022-03-06T10:57:39+00:00</lastmod> </sitemap> <sitemap> <loc>https://.../category-sitemap.xml</loc> <lastmod>2022-04-10T09:44:18+00:00</lastmod> </sitemap> <sitemap> <loc>https://.../post_tag-sitemap.xml</loc> <lastmod>2022-04-10T09:44:18+00:00</lastmod> </sitemap> </sitemapindex> * Connection #0 to host ... left intact <!-- XML Sitemap generated by Yoast SEO -->
Metadata for the included sitemaps does not seem to differ much. Content type is always text/xml.