Can't start dropbox on Ubuntu 18.04

I had the same problem (tried it also with a fresh installation, same thing). It turned out to be a locale issue (apparently, dropbox does not install well with some locales). You can check your locale by running

locale

and find out which locales are available for you by running

locale -a

I've managed to install dropbox properly after changing my locale to EN_GB.utf8 (was EN_IL beforehand) by running

sudo update-locale LANG=EN_GB.utf8
sudo update-locale LANGUAGE=EN_GB.utf8

I guess that you can change your locale back to what suits you best after the installation.

I opened language support, it asked me to install additional languages. Installed, restarted and it worked :)