Toad crashes when adding a new user, either when retrieving data from AD, or when adding a local user (4240166)

Toad crashes when adding a new user in the Server Security | Logins | Create New Login section, either when retrieving data from AD, or when adding a local user.

There are no errors in the application, but in the event viewer, it shows the following:

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Faulting application name: toad.exe, version: 6.8.1.204, time stamp: 0x5a0426a8
Faulting module name: mscorlib.ni.dll, version: 4.7.2110.0, time stamp: 0x597bd4f6
Exception code: 0xc0000005
Fault offset: 0x00000000004a5905
Faulting process id: 0x1d80
Faulting application start time: 0x01d3cb66dac3677c
Faulting application path: C:\Program Files\Quest Software\Toad for SQL Server 6.8\toad.exe
Faulting module path: C:\Windows\assembly\NativeImages_v4.0.30319_64\mscorlib\9aaf4ea9ded0a99b899b7bf4c971092d\mscorlib.ni.dll
Report Id: 90e76a37-b3ad-4b1f-9738-898d6c219f79
Faulting package full name:
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Application: toad.exe
Framework Version: v4.0.30319
Description: The process was terminated due to an unhandled exception.
Exception Info: System.AccessViolationException
   at System.String.wcslen(Char*)
   at System.String.CtorCharPtr(Char*)
   at System.Runtime.InteropServices.Marshal.PtrToStringUni(IntPtr)
   at Quest.Toad.SqlServer.Doe.Utils.ADPicker.ADPicker.RunDialog(IntPtr)
   at Quest.Toad.SqlServer.Doe.Utils.ADPicker.ADPicker.RunDialog(Quest.Toad.Db.IConnection, IntPtr, System.String)
   at Quest.Toad.SqlServer.Doe.Logins.LoginGeneralControl.lookupLoginButton_Click(System.Object, System.EventArgs)
   at Quest.Toad.SqlServer.Doe.Common.Controls.TextBoxAndButtonControl.RaiseOnButtonPressEvent(System.Object, System.EventArgs)
   at System.Windows.Forms.Control.OnClick(System.EventArgs)
   at DevExpress.XtraEditors.BaseButton.OnMouseUp(System.Windows.Forms.MouseEventArgs)
   at System.Windows.Forms.Control.WmMouseUp(System.Windows.Forms.Message ByRef, System.Windows.Forms.MouseButtons, Int32)
   at System.Windows.Forms.Control.WndProc(System.Windows.Forms.Message ByRef)
   at DevExpress.Utils.Controls.ControlBase.WndProc(System.Windows.Forms.Message ByRef)
   at DevExpress.XtraEditors.BaseControl.WndProc(System.Windows.Forms.Message ByRef)
   at System.Windows.Forms.NativeWindow.Callback(IntPtr, Int32, IntPtr, IntPtr)
   at System.Windows.Forms.UnsafeNativeMethods.DispatchMessageW(MSG ByRef)
   at System.Windows.Forms.Application+ComponentManager.System.Windows.Forms.UnsafeNativeMethods.IMsoComponentManager.FPushMessageLoop(IntPtr, Int32, Int32)
   at System.Windows.Forms.Application+ThreadContext.RunMessageLoopInner(Int32, System.Windows.Forms.ApplicationContext)
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