It looks to be when a member uses special characters in the member name like
it breaks the formatting on the rss feed and google reports that it is unreadable.
It is hard to nail down due to the feed is recent topics so it changes constantly.
Here is one error
Sitemap can be read, but has errors
We were unable to read your Sitemap. It may contain an entry we are unable to recognize. Please validate your Sitemap before resubmitting.