robbee82 on "[Plugin: Google XML Sitemaps] Monthly-based XMLs are empty"

Hello Mr. Brachhold,
it seems, there is a problem with the XML Generator. The sitemap.xml itself is available, also the uncluding sub-xmls. The only problem is, that aprox. the last 12 monthly-based XMLs are empty.
Furthermore the Webmaster-Tools shows up the following error message (in german): "Fehlendes XML-Tag". The error description contains "Dieses erforderliche Tag fehlt. Fügen Sie es hinzu und reichen Sie die Daten erneut ein."
Where can i start searching, to fix this issue? Is it a problem that comes along with the Yoast-Plugin (deactivated XML-Sitemap)?
Would be great if you could take a closer look on this.
Kind regards

https://wordpress.org/plugins/google-sitemap-generator/