Google Error with Sitemap after PHP/SQL upgrade

We use WordPress and PTO plugin and have moved to new servers with the following details:-

{code saved}

Only issue we identified during testing was with Featured Products not displaying which was fixed after applying node/509 fix.

However, Google is showing an error with every sitemap feed

"Errors
Missing XML tag
This required tag is missing. Please add it and resubmit."

Example

{link saved} only has display

This XML file does not appear to have any style information associated with it. The document tree is shown below.

Also, are there any other changes we need to do/consider for potential errors that we have not identified?

Many thanks

Jeff