You most likely have a note in your default note directory that has a wrong format and tomboy is crashing when it tries to process it.
By default, on Ubuntu, your tomboy note directory is located at:
~/.local/share/tomboy
Since you already have a backup of your notes, move the notes to a temporary directory such as:
$ mkdir ~/.local/share/tomboy.store
$ mv ~/.local/share/tomboy/* ~/.local/share/tomboy.store/
After this, you will have verification that tomboy is crashing because it's having a problem with an ill formated note. It shouldn't crash now, but should open up with no notes.
Now systematically move note from the store directory to the real directory. When you move the problem note tomboy will start to crash again. When you learn the problem note, you can eliminate that note and have a working tomboy.
You can copy your notes back 10 at a tome (or any number) by:
$ mv `ls ~/.local/share/tomboy.store/*note | head -10` ~/.local/share/tomboy/
You may have to quit tomboy and restart it between blocks to see if it still crashes, or to help verify where the culprit note lies.
By the way, tomboy's notes are identified by the *.note extension.
Also, please note. There is a bug with in Tomboy in that it will be confused with the timestamp of the note if you don't quit tomboy
between the moves (or copying of notes). It will change the timestamp
of the note to the current time if you don't quit. Quiting then
bringing in the new notes will preserve the correct timestamp.