I tried a complete reinstall of my RPi this morning using the most recent version of Raspbian “RASPBIAN STRETCH WITH DESKTOP, Image with desktop based on Debian Stretch, Version:November 2017, Release date:2017-11-29, Kernel version:4.9“. I then built the sample Qt5 project on Visual Studio 2017 to get everything back in place, including installing the Qt5 files on the RPi, which works fine but gives the same error messages.
I will try the alternative suggestion now.