Line 38 of xpra/trunk/src/etc/xpra/conf.d/55_server_x11.conf.in:
# -configdir ${HOME}/.xpra/xorg.conf.d
should be
# -configdir ${HOME}/.xpra/xorg.conf.d \
Otherwise the succeeding options aren't passed to xorg.
Fixed in trunk in r14522, r14523 for v1.0.x
Thanks for reporting this!
this ticket has been moved to: https://github.com/Xpra-org/xpra/issues/1378