Comment 4 for bug 1867766

I bet this is caused by this upstream change in librsvg:

- Librsvg's XML parser now supports namespaces (xmlns), and is
  stricter than before about it. Files may fail to parse if there are
  attributes or elements with namespace prefixes (e.g. foo:bar instead
  of plain bar), but without a corresponding namespace declaration
  (e.g. xmlns:foo="http://example.com/foo").

  This may happen especially with incorrectly-written SVGs that use
  xlink:href or xi:include attributes without the corresponding
  namespace declarations. If you run into this, just add the
  following to your toplevel SVG element:

      <svg xmlns="http://www.w3.org/2000/svg"
           xmlns:xlink="http://www.w3.org/1999/xlink"
    xmlns:xi="http://www.w3.org/2001/XInclude">
           ^^^^^^^^^ these ones

The nm-signal-*.svg icons in /usr/share/icons/ubuntu-mono-light/status/22 use sodipodi: prefixed attributes (exported by inkscape) without the corresponding namespace declaration (http://sodipodi.sourceforge.net/DTD/sodipodi-0.dtd).