Sorry, I was in such a hurry that I forgot to tell the version of
Epiphany. It's version 3.22.0.

I have changed the themes to test if the icons would appear.

# Before
Window: "Trisquel" (from one "trisquel"-related package???)
GTK+: "Trisquel" (from "trisquel-gtk-theme" package)
Icons: "Trisquel" (from "trisquel-icon-theme" package)
Cursor: "DMZ-White" (from "dmz-cursor-theme" package)

# After
Window: "Adwaita (default)" (don't know which package it came from)
GTK+: "Adwaita (default)" (id.)
Icons: "Gnome (default)" (from "gnome-icon-theme" package, or from
"gnome-icon-theme-full" package)
Cursor: "Adwaita (default)" (don't know which package it came from)

Still, the situation didn't change much, the icons are missing. The
error messages changed.