Are you using the newest version of iTunes (10.7)? Because I'm having a similar problem ever since I upgraded, using Windows 7 x64 too. I get the same message as your first example (session could not be started). I can sync my devices okay, but the backup portion fails. I can't run a backup separately either.
This has only been a problem since I upgraded to the newest version of iTunes, I had no issues the day before I installed it. I'm having the same issue for my iPhone and iPad, so it's not limited to one device.
I've also tried running through all of the recommended troubleshooting steps like uninstalling everything, setting up a new user, a new library, etc etc. I even tried downgrading to the previous version of iTunes, which didn't go quite as planned - it then wouldn't open my library, saying it was created using a new version of iTunes. The only thing that worked was backing the devices up on my Mac - backed up without a hitch. But that's not a solution as I don't use my Mac to sync my devices, and I don't have my library or media on it.
There's got to be some glitch with Windows and iTunes together - which wouldn't be uncommon for iTunes! As a temporary solution I'm backing up to iCloud.
EDIT: I should have mentioned that the only thing I haven't tried is restoring the devices - which I'm reluctant to do since I can't reliably back up the devices! iCloud just doesn't fill me with confidence for something like a critical backup.
Message was edited by: ColdFusionH3