I installed the latest Mac version of RPR (see details above) this morning and when I hit Shift+Z to get a render preview, I get an ERROR massage saying that I should check the log.
I've attached the log error. Weirdly, I get a perfectly good, fast render when I hit F12.
Late 2016 MBP running macOS Mojave 10.4.2 Beta (18C52a)
connected to 2 x Sonnet eGFX Breakaway Box each containing a Vega 64
The rendered viewport preview has worked well with all previous versions or RPR on this system.