One guess is that after the creation of new shelves the home view (in nickel) has changed in a way that creates troubles (nickel tries to open one of the png files that trigger fmon). If this is the case:
If you are running KSM 07, try
- tools > kobo diverse.msh > start nickel_no_fmon.sh
If you are running KSM 06, have a look at
this post, just for starting nickel, the other points that concern the data basebase might not be relevant, or maybe they are.
If you cannot start nickel in this way the database might be corrupted. You could try replacing the database by a back up.