Here's a response from one of the WPD technical people:
Hello Caro
Apologies for the delay as I have missed your email somehow. There is a bug with the sitemap plugin that we are aware and we are currently working to release a fix for it and instead use a different plugin.
For now, we do advice to :
Login to your wp-admin :
http://www.yourwebsiteaddress.com/wp-admin
Click Settings @ the right top bar
Then Click Settings
Click on Sitemap XML
Then Click Rebuild Sitemap
At the page there is an option to disable the auto generation of the sitemap which you need to check.
With Best Regards
Imed
Caro