I tried to add a couple of new feeds today and encountered an ERROR status after trying to sync them for the first time.
These both were WordPress based blogs, which I added using the /feed/ addition to the blog address.
It seems that WordPress is a bit sensitive to whether you have a slash (/) in the end of the address or not.
In one feed, I kept getting ERROR status until I added a slash at the end. In the second, I kept getting ERROR status until I removed the slash! So be sure to try both options if a feed is giving you trouble.
It seems the error lies with WordPress. The problem in both cases was that WordPress returned a malformed XML for the feed if the slash was given the wrong way…