Comment 1 for bug 1649428

Might be missing SSL support in some library or tool, or a failure to access that particular server - the file-uri plug-in can be built to use through gvfs, libcurl or wget, cf. https://git.gnome.org/browse/gimp/tree/plug-ins/file-uri/Makefile.am?h=gimp-2-8#n26

My bet for the Ubuntu packages is on gvfs, as the gimp package doesn't seem to have a dependency on libcurl or wget. I'm not running Ubuntu and thus can't check, though.