@emotionaltrash so yes I see the 503 error on the januari news articles sitemap for example... how many news articles did you post in that month?
I'm afraid adding simple pagination is not allowed in a sitemap and the plugin was built to split posts up by date, not by number. This was done to "keep it simple" as it uses WordPress's internal functions to get each month or year. I'd have to rethink the complete plugin structure to switch to splitting by number...