IIS 7.5 Manager crashes when adding a custom error page

I'm running a local IIS 7.5 server in Win 7 Pro, and I'm trying to add a custom error page for 403 responses.

When I click OK to add a custom error page for my site, IIS Manager just vanishes. The server is still running, and I can re-start IIS Manager, but the new page has not been saved.

I've also tried adding it directly to web.config, but that just gives me The page cannot be displayed because an internal server error has occurred.

Does anyone know why this might be happening?

Edit:
The event log implies that an invalid character in the path caused the crash, but It occured even when I copied & pasted a path from a valid entry.

Application error log:

IISMANAGER_CRASH

IIS Manager terminated unexpectedly.

Exception:System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---     System.ArgumentException: Illegal characters in path.
   at System.IO.Path.CheckInvalidPathChars(String path)
   at System.IO.Path.IsPathRooted(String path)
   at Microsoft.Web.Management.Iis.CustomErrors.CustomErrorsForm.OnAccept()
   at Microsoft.Web.Management.Client.Win32.TaskForm.OnOKButtonClick(Object sender, EventArgs e)
   at System.Windows.Forms.Control.OnClick(EventArgs e)
   at System.Windows.Forms.Button.OnMouseUp(MouseEventArgs mevent)
   at System.Windows.Forms.Control.WmMouseUp(Message& m, MouseButtons button, Int32 clicks)
   at System.Windows.Forms.Control.WndProc(Message& m)
   at System.Windows.Forms.ButtonBase.WndProc(Message& m)
   at System.Windows.Forms.Button.WndProc(Message& m)
   at System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)
   at System.Windows.Forms.NativeWindow.DebuggableCallback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)
   at System.Windows.Forms.UnsafeNativeMethods.DispatchMessageW(MSG& msg)
   at System.Windows.Forms.Application.ComponentManager.System.Windows.Forms.UnsafeNativeMethods.IMsoComponentManager.FPushMessageLoop(Int32 dwComponentID, Int32 reason, Int32 pvLoopData)
   at System.Windows.Forms.Application.ThreadContext.RunMessageLoopInner(Int32 reason, ApplicationContext context)
   at System.Windows.Forms.Application.ThreadContext.RunMessageLoop(Int32 reason, ApplicationContext context)
   at System.Windows.Forms.Form.ShowDialog(IWin32Window owner)
   at Microsoft.Web.Management.Host.UserInterface.ManagementUIService.ShowDialogInternal(Form form, IWin32Window parent)
   at Microsoft.Web.Management.Host.UserInterface.ManagementUIService.Microsoft.Web.Management.Client.Win32.IManagementUIService.ShowDialog(DialogForm form)
   at Microsoft.Web.Management.Client.Win32.ModulePage.ShowDialog(DialogForm form)
   at Microsoft.Web.Management.Iis.CustomErrors.CustomErrorsPage.AddCustomError()
   --- End of inner exception stack trace ---
   at System.RuntimeMethodHandle._InvokeMethodFast(Object target, Object[] arguments, SignatureStruct& sig, MethodAttributes methodAttributes, RuntimeTypeHandle typeOwner)
   at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture, Boolean skipVisibilityChecks)
   at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)
   at Microsoft.Web.Management.Client.TaskList.InvokeMethod(String methodName, Object userData)
   at Microsoft.Web.Management.Host.UserInterface.Tasks.MethodTaskItemLine.InvokeMethod()
   at System.Windows.Forms.LinkLabel.OnMouseUp(MouseEventArgs e)
   at System.Windows.Forms.Control.WmMouseUp(Message& m, MouseButtons button, Int32 clicks)
   at System.Windows.Forms.Control.WndProc(Message& m)
   at System.Windows.Forms.Label.WndProc(Message& m)
   at System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)
   at System.Windows.Forms.NativeWindow.DebuggableCallback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)
   at System.Windows.Forms.UnsafeNativeMethods.DispatchMessageW(MSG& msg)
   at System.Windows.Forms.Application.ComponentManager.System.Windows.Forms.UnsafeNativeMethods.IMsoComponentManager.FPushMessageLoop(Int32 dwComponentID, Int32 reason, Int32 pvLoopData)
   at System.Windows.Forms.Application.ThreadContext.RunMessageLoopInner(Int32 reason, ApplicationContext context)
   at System.Windows.Forms.Application.ThreadContext.RunMessageLoop(Int32 reason, ApplicationContext context)
   at Microsoft.Web.Management.Host.Shell.ShellApplication.Execute(Boolean localDevelopmentMode, Boolean resetPreferences, Boolean resetPreferencesNoLaunch)

Process:InetMgr