apt , did you solve your issue? Here, on an 1&1-Server (Ionos), I've got exactly the same issue:
Click on "GENERATE"
-> Popup appears "Sitemap-generation is in progress.." After a second, another, empty popup appears.
-> sitemap.xml file appears in root directory. It just contains xml-info, but no sites are added.
So, would be great to know if you were able to solve it.