The status page indicated that the sitemap was out of date, and since running cron with higher frequency did not improve the situation I went for a manual rebuild.
The rebuild took almost a day and not finishing, then I found that the sequential number of the processed node is higher than the count of all nodes. Something like this:
Remaining 10 of 14.
Now processing node 298652 (142409 of 114079).
That was on beta3, so I installed 6.x.-2.x-dev, ran /update.php, then I even deleted the sitemap via /admin/settings/xmlsitemap and ran the rebuild again. But I am in the same situation. The rebuild goes on forever and the number of the node processed is higher than the total count of nodes.
Ideas about what might be going on here?