Today I installed google sitemap generator and the XML file gets rendered (also the gzipped version).
I just have no clue why it does not include any static pages.
On the debug page there are not any errors showing up.
Another problem is that the priority values are floored to integers. Which made the startpage 1 and all other entries 0. I still can select for e.g. 0,6 from the dropown but it gets converted to 1. I saved all items with value 1 but still no static pages showing up in the sitemap.xml.
The INT problem might be caused by my locale (which is de_DE). You can see in the sourcecode that the numbers of the priorities JS array are written as 0, 0,1, 0,2, 0,3, etc. instead of 0, 0.1, 0.2, etc. I changed this one line manually but it did not have any effect. I don't know the plugin well enough to dig any further.
Removing the local localizations and so switching back to english did not help either.