Android playback/compatibility issues have been a moving target since its introduction. Each vendor has slightly different platform modifications and different issues are found on different devices. If you read back a ways, you'll find a long thread on the difficulty getting the Droid phones working. They always seemed to have problems.
That said, the Samsung devices have had a pretty good track record and I haven't seen your particular issue raised previously. The fact that it's just the ATT version that your having trouble with, suggests that there must be a minor variation with that implementation that is gumming up the works.
A second thought is that the tablet is running Android 3.2. Considering that the SS Android app has been updated a few times to try keeping pace with the different Android versions, you might have better luck with an older version of the app. You can find earlier versions here:
http://sourceforge.net/projects/subsonic/files/android/There are also a couple of developers that have more updated versions of the app posted in the forums that have attempted to achieve better compatibility with ICS. There's a chance that one of those may also work better than the current app available from Play Store.
I'm currently using "Subphonic" on my Galaxy SIII and it seems to be working fine.
The final thought would be that on a 9" tablet you should be able to just access your server through a browser window. I use Dolphin on my SIII to handle the video playback through the app interface and it works well.
HTH