In my /category-sitemap.xml I noticed that the last entry is a post URL, not a category URL.
This same post is as normal included -also- in the /post-sitemap.xml
This post belongs to a category that has only one post, the one in question.
This category is as expected mentioned in the /category-sitemap.xml only once but -and that's the problem- just as part of its unique post URL.
This category has no child category; it has one parent category.
In other words, in my /category-sitemap.xml there is a false entry "/category/post" instead of "/category/" for a category that has just one post, one parent category and has no child category.
WPSEO version 18.104.22.168
Strip the category base: enabled
URI structure: /%category%/%postname%