I was testing a multiscreen problem and ran the following,
Xephyr :2 -screen 640x480 -screen 640x480 -screen 640x480
qdbusviewer didn't show and gave an error,
QXcbConnection: XCB error: 153 (Unknown), sequence: 154, resource id: 0, major code: 145 (Unknown), minor code: 20
It has the same problem if I only opened one screen (in which :2.1 crashed :2.0 didn't crash, the crash would be a different problem). Start a fresh Xephyr connect no other clients, run qdbusviewer, doesn't display, kill and run qdbusviewer again, doesn't display. Display xeyes to Xephyr, then run qdbusviewer, doesn't display, kill and run it again, it now displays as long as there is at least one client that keeps Xephyr from resetting, kill that client and again it doesn't display. I looked a little bit at the X protocol with wireshark, which shows problems, but it wasn't clear if it was an Xephyr bug or Qt5 bug, though other programs don't seen to have a problem.