Update (29-Jun-11 15:59 GMT): We've put in place a workaround and have gotten confirmation from a number of users that it alleviates the problem. If you you experience any errors, please let us know on Twitter (@spanningsync), on our forum, or via email to firstname.lastname@example.org. Thanks!
We're continuing to hear from lots of users who can't log into Spanning Sync. When the Spanning Sync client tries to fetch a list of the user's calendars, it's getting a generic "Spanning Sync encountered an error" error. Under the covers, our authenticated request to Google for the user's metafeed (list of calendars) is getting a "400 Premature end of file" error" from Google.
The problem seems to affect some users consistently, some sporadically, and others not at all. I've posted a request for technical support from Google to the Google Calendar API forum and am optimistic Google will find and fix the problem on their end soon. You can follow that discussion here: http://goo.gl/tX1xN