So. It turned out that the error was indeed due to lack of setting the paths :
gtk-sharp from source ,
I put these additional lines in the module file for aspect/0.8 :
prepend-path PATH /c3se/NOBACKUP/users/negrato/Soft/gtk-sharp/gtk-sharp-2.12.45-install/bin prepend-path LD_LIBRARY_PATH /c3se/NOBACKUP/users/negrato/Soft/gtk-sharp/gtk-sharp-2.12.45-install/lib
where the paths are pointing to the installation directory. Then ‘monitor’ tool will work.
For completeness, I still see the following error popping up in the terminal:
Error, libnotify.so.4 was not found, notifications won't be displayed.
but the tool works just fine.