Hi Rik,

looking at your problem it could be related to a scalability issue. Maybe having so much links the process exists before completing leaving the XML broken.
Also if your links exceed 50,000, you should activate the splitting feature before exporting it.

You should:
1. Enable splitting for example 30,000 links for each file
2. Precache the Kunena topics, etc
3. Export the splitted sitemap

If you have still the problem, so maybe there could be really some invalid characters somewhere, but i should look at it deeply to tell you more.

Best,
John