I've been playing quite a bit with Excel Services the last few weeks, and ran into a problem around refreshing data in a worksheet stored in Excel Services when I used Single Sign On (SSO) in the ODC.
I also noticed that each time my report failed to refresh data, the error in the subject of this post got dumped into the event log.
I asked for help, then did some digging. Turns out this error is one of those "by design" (or, "by bad design") issues. Some of you might call it a bug :)
In essence, if the account you use for the Shared Services Provider (as in "SharedServices1") is different than your Farm and/or SSO Account identities, you'll get this error. They must all be the same, even though I didn't see this documented anywhere (and to be fair, I haven't spent tons of time looking).
Thanks to Chris Nicholas for his help on this problem!