Hi Ernst,
I don't think they are particularly hidden 
The location does vary according to operating system  but he easiest way to locate the log folder is to use the Qimage Utilities menu  and select "Explore Qimage Application Data". You will see the Log folder in the list.
Where there is a copy of Q-SE and Q-U on the same system, both share the same Application data so which ever version is used, the log should be updated.
I now have a new PC with only Q-U installed but the old XP machine had both Q-SE and QU and I know the Application data was shared without any problems because I did recall some old Q-SE jobs from within Q-U.
Terry