Please advise- GSC treating my valid pages as Soft 404

Google Search Console has treated almost all of my valid pages as Soft 404. It has also rejected my valid sitemap (/sitemap_index.xml) saying that " Your Sitemap appears to be an HTML page. Please use a supported sitemap format instead. "

I can access all my pages well. No penalties whatsoever. SSL certificate is ok. My content is long-form (more than 4,000 words) and original. I've contacted Namecheap (hosting) and they say everything checks out on their end. I have tried resubmitting my pages for indexing but GSC still says " Page cannot be indexed: Soft 404 ", yet the pages can be accessed as usual.

As a result, most of my pages have disappeared from Google's SERPs

Anyone with a possible solution?

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Thanks