It’s possible for this io variable to contain just about anything, and as long as it isn’t obviously XML, then it will try to open the string as a file:

# lib/rfm/utilities/sax_parser.rb:838
        def run_parser(io)
          parser = Nokogiri::XML::SAX::Parser.new(self)
          parser.parse case
                       when (io.is_a?(File) || io.is_a?(StringIO))
                         io
                       when io[/^</]
                         StringIO.new(io)
                       else
                         File.new(io)
                       end
        end

I'm still not sure what Filemaker was responding with, since restarting the WPE fixed the problem, but it was really confusing to see Errno::ENOENT in multiple apps' log files.

If this happens again I'll take a closer look and hopefully make a pull request to at least make the error more descriptive.