I was getting this same error for an iPhone and two iPads after updating to iTunes 10.2.1, and installing Xcode 4 under Mac OS X 10.6.6.
Here's what I did to resolve the issue (NOTE: These steps came directly from an Apple Support document which outlines removing iTunes completely from your computer. However, since I wasn't willing to do this ALL the outlined steps, I only applied the steps below):
1. Disconnect any iOS device(s) from the computer.
2. Move the iTunes application icon to the Trash.
3. From the Finder, choose Go > Go to Folder
4. Enter (without quotes) "/System/Library/Extensions" and then click Go.
5. Locate the file "AppleMobileDevice.kext" and drag it to the Trash.
6. Choose Go > Go to Folder.
7. Enter (without quotes) "/Library/Receipts/" and then click Go.
8. Locate the file "AppleMobileDeviceSupport.pkg" and drag it to the trash. Note: This file may not be present in Mac OS X v10.6 or later; skip to the next step if this is the case.
9. Restart your Mac.
Since I was very nervous about losing data, I did NOT empty the Trash.
After restarting, I downloaded the latest version of iTunes and installed it: http://www.apple.com/itunes/download/
Once complete, I opened iTunes and was quite pleased to see that all my data was intact. I then plugged in an iPad and voila -- problem solved. iTunes did not generate the "iTunes was unable to load provider data from sync services" error and it completed a backup, then sync'd without error. I did the same for my iPhone and finally my wife's iPad.
Hope this helps!
Yes it solved it for me too, so many thanks.
I don't know exactly what caused the problem: I installed iTunes 10.2.1, the Java update, upgraded MobileMe calendars, and applied the Xcode update at about the same time, but then when I plugged either in I immediately got the message in iTunes "iTunes was unable to load data class information from Sync Services. Reconnect or try again later." Except it didn't, and neither did restarts or running Onyx.
I noticed that Windows users tend to get this message a lot, but yours is the only I could find for Mac Users.
So in short reinstalling iTunes did the trick for me. I deleted the AppleMobileDevice.kext from /System/Library/Extensions and AppleMobileDeviceSupport.pkg from /Library/Receipts/ by hand in the Finder though, although you have to enter your password. I'm not sure whether these steps or essential or not.
So many thanks again David, you've made me a very happy man.
I also got this message after installing Xcode 4
All I did was download iTunes 10.2.1 from Apple and re-install it. Specifically:
1) Disconnect iPhone
2) Quit iTunes
3) Open downloaded iTunes 10.2.1 dmg and double click Install iTunes
4) After iTunes installed, connect iPhone.
Problem was gone.
So, I think re-installing iTunes solves the problem, but I'm not sure you need to go through all the steps of finding the extensions, receipts, etc.
I just had the same problem after installing XCode.
I was constantly getting the "itunes was unable to load data class information from sync services" message whenever I selected my phone in the Devices sidebar. In addition, the "devices" tab in iTunes preferences had no entries for either of my phones, so even after the sync was completed, it complained that "itunes could not backup the iphone because the backup session failed".
I just re-installed iTunes 10.2.1, without bothering to touch the extensions/receipts, etc., and things are back to normal now. The entries for my phones re-appeared in the devices tab of iTunes preferences, and all sync sessions now go through completely without any errors.