After my inital post here I thought I resolved the issue.
However upon boot up this morning the error reoccured. Odd thing is my backup did seem to backup and my new music I copied to my iTunes library did copy to my iPhone 4s.
So bottom line it appears hit and miss that the aforementioned fix actually resolves the issue long term.
BigNavy505 post from 9/19 @ 5:17pm worked for me.
Couple things to remember when doing that...
1) Turn off Sync with WiFi (You MUST disconnect both the WiFi AND wired connections for the change to be effective. Shutting down ITunes does NOT disconnect your IPhone from your WIFI)
2) Turn off your WIFI connection on your IPhone (see above)
3) Disconnect your IPhone from the PC and shut down ITunes
4) Start ITunes after making the changes
5) plug in IPhone
This would not resolve the problem because it is not a 64-bit Windows problem. As I mentioned earlier, I have a 32-bit machine and I have the same problem.
However, I have noticed, as Erik pointed out, above, that WiFi synching still works. It's the USB syncing that does not work properly. And also, as a few people have noticed, sometimes the error message pops up but it did actually sync anyway. But this is not always the case. And although a few of the "fixes" that people have found here seem to work initially, the next time you do a sync, the "fix" no longer works.
It seems that the best work-around fix for the moment, until Apple actually fixes this, is to sync over WiFi.
OK, I have been having the same problems as everyone else in this thread. I have Windows 7 Ultimate, 64-bit, and since the iTunes update synching was having multiple errors. After a lot of experimentation I have fixed my synch issues by doing the following:
1. The inability to back-up when connecting the iPhone 4S to itunes via USB cable:
-This was fixed by resetting the lockdown folder. Steps found at http://support.apple.com/kb/ts2529
-Be aware that after resetting the lockdown folder, you will need to input your passcode into your phone (if you lock your phone with a passcode) the first time you reconnect your phone via cable
-After the lockdown folder is reset, connecting via the cable allows backups to function again. The error does not return after restarting iphone, itunes, or computer, so I think that permanently fixes the error
2. The inability to sync apps when connecting the iPhone 4S to itunes via USB cable:
-This is the bigger issue IMO. I had noticed that apps were not synching between my computer and iphone even when synching over Wi-Fi, even though no error messages were displayed.
-After MUCH experimentation, the only way I got this to work was to disconnect the iphone from my computer, sign out of my Apple store account on BOTH my iPhone and itunes (on my computer), restarting BOTH the iPhone and iTunes, signing BACK onto both, and then reconnecting the iPhone to iTunes. Note that the first time I reconnected them after signing back in, the error still persisted. However, unplugging them and then plugging them back in synched apps for the first time since the problem came up, so I have hope.
I am confident that the backup issue is fixed by following the steps noted in #1 above. I'm not as confident about #2 fixing the app synching issue, but it's working for now.
I hope this helps, at least until Apple fixes whatever they did to this app.
FYI I'm running iOS 5.1.1, the last iteration before iOS 6.0.
I'm not having any trouble with backing up - only with synching - so I didn't do your first step. But I did the second step ...to no avail. When you say "However, unplugging them and then plugging them back in synched apps..." What are you referring to? I unplugged the iPhone from the USB port on the PC and then plugged it back in (which didn't improve anything)...but what else did you unplug?
Unfortunately deleting the lockdown folder doesn't fix the problem for me. It temporarily allows me to backup my device, but eventually the error returns.
I'm not having any issue with syncing - the issue for me is backing up. The trouble is that this error message obviously has different causes, making it hard to figure out if we're all having the same problem. Regardless, there is obviously some issue with the current version of iTunes.
As others have pointed out, Wi-Fi syncing doesn't seem to be affected. I've successfully backed up and synced my devices over Wi-Fi while the error was occuring when connected by USB.
I upgraded my devices to iOS 6 last night - waiting to see how things progress from here, in case it's somehow fixed the problem. Too soon to tell so far!
This is a link to restarting the Apple Mobile Device Service. As soon as this done, backups work just fine. However, it seems to be temporary as I've needed to restart it every so often. Temporary solution until Apple gets this very crippling problem resolved.