I got the following error message when opening the sitemap:

Fatal error: Call to a member function setState() on a non-object in /home/haripena/public_html/modules/mod_related_items/helper.php on line 44

I disabled the module Related Items and the problem was fixed.

I suppose that there is a conflict between the new version of the Related Items Module and JSitemap. As a temporary workaround, I replaced the file helper.php with the one from the previous Joomla version. Everything is working well so far.