On a fresh install of WordPress, after installing the P2 theme, you will get this error if you delete the default post and try to visit the page:

ERROR: This is not a valid feed.

You can also see this error as a HTTP 404 for a request to /feed/p2.ajax or ?feed=p2.ajax (depending on your settings).

The issue is caused by the missing Post. Simply restore it from the trash and you should be good to go.