When Qt-based applications are started in XFCE 4.10 GIT : they hangs with
a white window. Terminal outputs vary :

"GuiToolbar.cpp(339): Session settings could not be found!. Defaults are used
instead"

or

"X Error of failed request: BadLength (poly request too large or internal Xlib length error)
Major opcode of failed request: 148 (RENDER)
Minor opcode of failed request: 17 (RenderCreateGlyphSet)
Serial number of failed request: 8829
Current serial number in output stream: 8842"

But these same applications start properly on Gnome, on the same machine.

Installing xfce-settings-4.8.3 instead of xfce-settings-4.10 solve the Qt problem (confirmed by several testers on several different hardware).