Same issue again today. The updater window opens, the "checking" icon spins briefly, and the window crashes. The following entry appears in the Windows Application log:
Application: SkylordsRebornUpdater.exe
Framework Version: v4.0.30319
Description: The process was terminated due to an unhandled exception.
Exception Info: System.IndexOutOfRangeException
at SkylordsRebornUpdater.MainWindow.GetUpdateFile()
at SkylordsRebornUpdater.MainWindow.CheckForUpdates()
at System.Threading.ThreadHelper.ThreadStart_Context(System.Object)
at System.Threading.ExecutionContext.RunInternal(System.Threading.ExecutionContext, System.Threading.ContextCallback, System.Object, Boolean)
at System.Threading.ExecutionContext.Run(System.Threading.ExecutionContext, System.Threading.ContextCallback, System.Object, Boolean)
at System.Threading.ExecutionContext.Run(System.Threading.ExecutionContext, System.Threading.ContextCallback, System.Object)
at System.Threading.ThreadHelper.ThreadStart()
As the game was working fine for many months, I'm sure my system has the necessary prerequisites. I did, however, follow all of the relevant suggestions in this thread, including disabling firewall/anti-virus, deleting Documents\BattleForge, deleting the entire application folder and downloading and extracting both the client and updater again, restarting several times, etc.