Preview server produces only error msg

localhost:4567 displays only “Files will be uploaded in a couple of minutes.” after preview server is started.

Running Linux Mint 18.1 Serena and middleman 4.2.1

I’m pretty much a rank amateur, but have used middleman successfully and with great pleasure for about two years. Returning after about 6 months away I get this.

I can find no info. on this error anywhere. Totally stuck. All suggestions most appreciated!

Tom