Resolving XML Sitemap Errors in Google Webmaster Tools

Hi there,

This support request is in regards to Apologies on the lengthy description, but I just want to be thorough because I'm fairly lost. The client's SEO company asked us to fix the XML errors identified in the client's Google webmaster tools panel.

First, we generated a sitemap using WordPress SEO for Yoast, identified as /sitemap_index.xml. I then submitted this to Google Webmaster Tools in the Webmaster account panel. I noticed that there was a previously existing sitemap also added into the panel, identified as /sitemap.xml. Should I delete the /sitemap.xml or leave it in there? It doesn't make sense to me to have two...

Anyways, both have a significant number of "errors" listed. Google Webmaster gives me a URL for the location of the error, and for one in particular it says it appears 116 times (!), but I dont recognize the URL and it just kind of "looks funny". If you click on the link it takes you to a 404 page.

Could you please let me know what these errors are and how to fix them? I am somewhat new to this, any advice would be greatly appreciated thank you so much!

Kind regards,