No errors or warnings, but the resulting capture is blank.
All distributions can save a PNG screen capture, except for Ubuntu 16.04
Re-tested all other current Ubuntu versions (14.04, 16.04, 18.04 and 18.10), only 16.04 is affected. Fixed a bug when xshm is disabled: #2143, but disabling xshm does not help. Using the GTK root window capture code tool added in r21645 does not work either.
Strange thing is that xwd does capture the X11 display without problems.
Anyway, turning off "3d acceleration" in virtualbox fixed the problem. Clearly a driver bug.
this ticket has been moved to: https://github.com/Xpra-org/xpra/issues/2063