[REG 4.8.6->5.5] QTimer doesn't work while screen is locked [works when qt configured with -no-xcb-xlib]

Qt 5.5 QTimer doesn't work while the session / screen is locked by Light Locker (Xubuntu, standard configuration). I have attached two sample apps that shows the problem. A version for the console and one with GUI.

The console version works when the screen is locked, the GUI version is stopped as long as the screen is locked (see the missing timestamps in the output).

Qt 4.86 works fine
Qt 5.5 console: ok
Qt 5.5 GUI: fails