Missing XML tag on line 3

2 of my sitemaps in my sitemap index gets the “Missing XML tag” error in Google webmaster tools, it points at:

Line: 3
Parent tag: urlset
Tag: url

If I go to the sitemap in the browser it shows an empty urlset:

<?xml version="1.0" encoding="UTF-8"?><?xml-stylesheet type="text/xsl" href="https://www.example.com/wp-content/plugins/google-sitemap-generator/sitemap.xsl"?><!-- sitemap-generator-url="https://www.arnebrachhold.de" sitemap-generator-version="4.0.8" -->
<!-- generated-on="2016-01-14 11:11" -->
<urlset xmlns:xsi="https://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="https://www.sitemaps.org/schemas/sitemap/0.9 https://www.sitemaps.org/schemas/sitemap/0.9/sitemap.xsd" xmlns="https://www.sitemaps.org/schemas/sitemap/0.9"></urlset><!-- Request ID: xxxxxxxxx; Queries for sitemap: 8; Total queries: 79; Seconds: 0.01; Memory for sitemap: 0.5MB; Total memory: 18.5MB -->

Why does the error occur and how can i resolve it?

https://www.ads-software.com/plugins/google-sitemap-generator/