Andrea Greselin <greselin.andrea@gmail.com> writes:

> Note that if I remove '(recentf-mode)' from the init file and execute it in
> Emacs instead (doesn't matter whether interactively or by evaluating
> Elisp), the widgets work even in a client session.

I guess the problem is that tree-widget-image-enable is set just once on
load, based on the current display (and the daemon has no display so it
doesn't support displaying images).  Does it help to explicitly set
tree-widget-image-enable to non-nil?