Sitemap and index error

Hi,

After the latest Yoast Update, I see an HTTP error in GSC for this sitemap: https://www.vrouwenannonu.nl/post-sitemap.xml

Also, when I want do index a separate URL, I get an error:
Indexation request declined
Indexing issues with the URL were found during the live test

Live test:
URL is unknown to Google
URL is indexed only when certain conditions are met

Same problem with my other websites after the update.