Ok finally figured it out! I had to manually copy all the provisioning profiles from my development machine ("~/Library/MobileDevice/Provisioning Profiles") to the Xcode Server machine ("/Library/MobileDevice/Provisioning Profiles"). Really can't believe how buggy the Xcode Server is, since it should've done that automatically as soon as the server is added to the developer team in the Xcode Server settings.
Anyways, please download the attached zip file and follow these steps carefully:
1. On your server machine, unzip the file then move the folder called "XcodeServer" to your STV package, under the folder named "Plugins".
2. From Finder, navigate to the "STV 4.x/Plugins/XcodeServer" folder and double-click the file called 'InstallSTVServerPlugin.command'.
** IMPORTANT NOTES **
- Unlike the regular STV plugin, the STV Xcode Server plugin must be reinstalled each time you upgrade Xcode on your server.
- If for any reason you face problems after installing STV's Xcode Server plugin, you can uninstall by double-clicking the file called 'UninstallSTVServerPlugin.command'.
Please let me know if this fixes the problem for you.
Edited by Tarek, 15 April 2015 - 10:21 PM.