It seems my last version of the Xml Sitemaps module for pligg didn't really fixed the date format problem with the generated sitemaps.

Back when this module was created google had less strict rules about the date format in the lastmod section. My module generated a date and time string with this format YYYY-mm-ddTHH:MM:SS and it was ok but now it's only valid if it also contains the timezone offset in this format +/-HH:MM or if the string doesn't contain the time anymore.

So here is another update on this module that also contains the timezone offset so the sitemap is considered valid by google.

Download here