I had a server open on display :2, but issued a command
xpra stop :1. The response was:
InitException: cannot find any live servers to connect to xpra initialization error: cannot find any live servers to connect to
The correct minimal response should have been to the effect that no live server was found on the requested display. A pampering, hand-holding, supportive, millennial response might also list the existing live servers.
Fixed in r17523.
$ xpra stop :10 xpra initialization error: cannot find live server for display :10
this ticket has been moved to: https://github.com/Xpra-org/xpra/issues/1711