While upgrading GroupShare 2015 to GroupShare 2017 SR1 I encounter a fatal error.

I've carefully followed every step of the Installation Guide, and there are no problems during the initial install phase (installing the application) but when the installer moves on to upgrade the database (I've chosen the "Use existing Microsoft SQL Server and configure automatically" option) I encounter...

System.Security.Cryptography.CryptographicException: Specified initialization vector (IV) does not match the block size for this algorithm.
   at Sdl.Distribution.ServerInstall.Wizard.PostInstallStep.<DoConfiguration>b__3_1(Object sender, RunWorkerCompletedEventArgs args)
   at System.ComponentModel.BackgroundWorker.OnRunWorkerCompleted(RunWorkerCompletedEventArgs e)
   at System.ComponentModel.BackgroundWorker.AsyncOperationCompleted(Object arg)
   at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
   at System.Windows.Threading.ExceptionWrapper.TryCatchWhen(Object source, Delegate callback, Object args, Int32 numArgs, Delegate catchHandler)
   at System.Windows.Threading.DispatcherOperation.InvokeImpl()
   at System.Windows.Threading.DispatcherOperation.InvokeInSecurityContext(Object state)
   at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
   at MS.Internal.CulturePreservingExecutionContext.Run(CulturePreservingExecutionContext executionContext, ContextCallback callback, Object state)
   at System.Windows.Threading.DispatcherOperation.Invoke()
   at System.Windows.Threading.Dispatcher.ProcessQueue()
   at System.Windows.Threading.Dispatcher.WndProcHook(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
   at MS.Win32.HwndWrapper.WndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
   at MS.Win32.HwndSubclass.DispatcherCallbackOperation(Object o)
   at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
   at System.Windows.Threading.ExceptionWrapper.TryCatchWhen(Object source, Delegate callback, Object args, Int32 numArgs, Delegate catchHandler)
   at System.Windows.Threading.Dispatcher.LegacyInvokeImpl(DispatcherPriority priority, TimeSpan timeout, Delegate method, Object args, Int32 numArgs)
   at MS.Win32.HwndSubclass.SubclassWndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam)
   at MS.Win32.UnsafeNativeMethods.DispatchMessage(MSG& msg)
   at System.Windows.Threading.Dispatcher.PushFrameImpl(DispatcherFrame frame)
   at System.Windows.Threading.Dispatcher.PushFrame(DispatcherFrame frame)
   at System.Windows.Application.RunDispatcher(Object ignore)
   at System.Windows.Application.RunInternal(Window window)
   at System.Windows.Application.Run(Window window)
   at Sdl.Enterprise2.Installers.EnterpriseEdition.Program.Main(String[] args)

The first time this happened I thought it might have been due to some steps which I hadn't been certain about as I was performing them (for example, I wasn't certain that the Powershell scripts had run correctly) so I restored both the application and database servers from backups and started all over again, this time ensuring that each step was done precisely. And I got the same error both times.

Upon clicking "OK" on the Fatal Error dialog both disappear and the installation is brought to an abrupt halt. Of course, GroupShare 2017 is now partly installed but is unusable. Yes, I can revert to a backup of the previous (2015) installation but I'm unable to upgrade to 2017.