I have two multi-core computers, both run Ubuntu 16.04 with octave 4.2.0 installed. One is a server that has very elementary VGA support, processes are normally run through "ssh -X IP.add". This is the system that gave the error I reported in my original message. If I install a local terminal and run octave, then qt plotting is fine (but so hardware-limited as to be useless). Perhaps this is what Kal meant when he was asking whether this was a "remote" machine. I though he meant was the machine off-site.
The other system has conventional video driver and monitor. I have experimented with this issue with and w/o mpi, and conclude that mpi has nothing to do with the problem, my error. However, if I try to ssh into that system, qt fails but in a totally different way: no error is logged, as in the original post, and a figure frame appears, but apart from the menu bar, it consists entirely of whatever image lied under the frame, where it first appeared.
My general conclusion is that qt fails when accessed through ssh, even though other graphics packages (gnuplot, ncview) work fine, and the mode of failure is variable. You may have already known this, in which case sorry for the bother.