Thanks a million, maacruz :-)

Your tips brought me on the right track.

The reason the SSU was broken was that I had previously created a symlink in /home/user/MyDocs to a directory on the external memory card.

After deleting that link 'apt-get -f install' finally completed successfully.

Jesus, what is wrong with creating a symlink in my MyDocs directory?

Not to mention the trouble with Modest, which eats so much CPU/battery that I always have to shut down the N800 to prevent the battery from becoming depleted.

All this hassle made me buy an iPod touch, which is a real joy to use compared to the N800 after it was hacked kaputt by Nokia's Diablo + SSU.