The problem is that View - Show Output Window is available only in UltraCompare Professsional. In UC Lite there is no menu item to make the output window visible again after closing it. I reported this issue now by email to IDM.
The workaround is to enable the output window manually in the registry. Open in UltraEdit a new ASCII file with DOS line terminations and copy and paste following lines into this new file.
[HKEY_CURRENT_USER\Software\IDM Computer Solutions\UltraCompare Lite\Workspace\BasePane-30723]
Save the new file for example with the name UCL_Output_Window.reg. Important is the file extension REG. Switch to Windows Explorer and double click on this registry file to import it into your registry. As you can see it enables visibility of a pane, the pane of the output window. Next time you use UCL the output window is visible again.