I don't know exactly what goes wrong with zim when started by debbie - it gets quite confused about the notebook structure, which is defined in /home/user/.config/zim and than with further control files in the notebook directories defined therein (which I put under MyDocs).
With "sudo debian su -c zim - user" everything works as it should, but not when starting via debbie, although zim does find its folders initially. But when using it further, it does not keep track of what it's doing, and ends in a mess.

I noticed that "su - user" leads to a pwd of /home/user, but "debbie" has / . Also /home/user/.profile is only read with "su - user", hence my suspicion that some things are not initialized like in a regular login shell.