Hi
I had the same problem and finally today I’ve fixed. https://www.gincanaland.com/category-sitemap.xml
First if you have installed W3 Total Cache: under Browser Cache, try to disable “Set W3 Total Cache header” for HTML & XML and clean all caches.
But for me that wasn’t enough, I had to fill all boxes of Yoast WordPress SEO Categories in Edit Category page for all categories including uncategorized. Besides, I had to check the 2 last combos without using default values.
After that, I save changes in Sitemaps XML page. I hope it works for you too. Good luck