I just installed 2.6.1 (deleted and installed fresh from 2.6). I am still getting "Communication Error, 500 Server Error" and "Operation could not be completed. NSXMLParserErrorDomain error 5" for one of my blogs (http://www.dalcher.com/blog).
Another one of my blogs (http://www.stupidranger.com) is working just fine.
Any troubleshooting advice you could provide would be appreciated.
EDIT: Please disregard. I deleted and re-added the site configuration and it seems to have resolved the issue. It seems only to have thrown the errors after the first configuration of the site after a clean installation. Thanks!