QTconcurrent & XInitThreads problem

[gfxx]

I launch from a gui with kplot a libmodbus connection that give to my applicatio data to plot.

to make the connection and receive the data I use qtconcurrent to go in parallel the guidance and reception.

qt applicatio run on ubuntu12.04 an I use -lX11 and XInitThreads() funtion to facilitate the use of qtconcurrent ... but the debug give me this error:

@[xcb] Unknown request in queue while dequeuing
[xcb] Most likely this is a multi-threaded client and XInitThreads has not been called
[xcb] Aborting, sorry about that.
miapplicazioneQT: ../../src/xcb_io.c:178: dequeue_pending_request: Assertion `!xcb_xlib_unknown_req_in_deq' failed.
Aborted@

Can you tell why