Google Indexing cURL 28 error

I have a website in Wordpress and I am showing a sitemap to google with about 6,800 urls.

Some days ago there were 6,750 urls indexed. Today there are about 500.

Webmaster tools reports:
5,000 urls sent and indexed (this has been constant for a while now)
1,800 discovered and not indexed: Excluded

The excluded urls are not blocked in robots.txt or have a noindex follow tag. I tested some of them in google page speed insights and it reads the urls.

SEO Yoast health tool sais there is a...

Google Indexing cURL 28 error

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