Hi @NicolasK,
This is a known issue with Google Search Console. Google doesn’t provide any logs about this and for random reasons keeps saying couldn’t fetch sitemap. If you wish, you can try to contact Google about this, but as long as your sitemap is working, which I can see it is, the issue is on Google’s side.
I’ve experienced this myself on few of my sites and all I could do is wait indefinitely until Google decided to check my sitemap again and saw that it was working.