After upgrading to Remote Desktop Manager 13.5.x, you may receive the error below;
RootException - Root is empty!
at Devolutions.RemoteDesktopManager.Managers.RootConnectionManager.get_RootConnection()
at Devolutions.RemoteDesktopManager.Frames.ConnectionViews.FreConnectionTreeListView.ce5f4c5875531b613e87da05d31efd852(ConnectionViewMode cfa8984eaceb595fb57911c4e0ee96824)
at Devolutions.RemoteDesktopManager.Frames.ConnectionViews.FreConnectionTreeListView.LoadConnectionList(Connection[] connections, ConnectionViewMode viewMode)
at Devolutions.RemoteDesktopManager.Forms.FrmMainRibbonBase.LoadAllConnectionView(Boolean saveState)
at Devolutions.RemoteDesktopManager.Forms.FrmMainRibbonBase.RefreshAllConnectionView(Boolean saveState, Boolean checkOnline)
at Devolutions.RemoteDesktopManager.Managers.MainFormManager.DoFirstLoad(IMainForm mainform)
at Devolutions.RemoteDesktopManager.Forms.FrmMainDocumentManager.cae3b4c7c167cf0d9747ac4fee11ac00a(Object c19f185fd70cefc696ba148af1c4faf54, EventArgs cf1018bb83ec7debd818319fd3cb4844e)
at System.Windows.Forms.Timer.OnTick(EventArgs e)
at System.Windows.Forms.Timer.TimerNativeWindow.WndProc(Message& m)
at System.Windows.Forms.NativeWindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)
Many issues related to it have been addressed in the latest version which you can download here;
https://remotedesktopmanager.com/home/download
The other solution is to simply switch between data sources or delete and recreate the data source in File -> Data Sources.