Hi,
For the problems with the -archives.xml: For performance reasons, the plugin currently does not check if you have any posts or not. If this entry causes a problem, you can deactivate the archives in the cotnrol panel of the plugin.
For the problems with the -externals.xml: Can you try to go to the sitemaps control panel and click the save button at the end of the page? Also make sure there are no empty lines "Additional Pages" section.