Comment 2 for bug 1853754

This is an Dropbox upstream issue. There is a work-around:

Caja dropbox does not start.

The libdropbox_apex.so shared object, as distributed by Dropbox, has the wrong file permissions.

The workaround is to open a terminal and run the following command:

sudo chmod a+rx /var/lib/dropbox/.dropbox-dist/dropbox-lnx.x86_64-*/libdropbox_apex.so