I am replying to myself.
The problem is not PowerDVD but the Onkyo receiver.
TO ANYONE HAVING TROUBLES WITH 3D THROUGH HDMI 1.4 RECEIVER (plugged to HDMI 1.4 3D TV too of course)
I suspect the last onkyo firmware to be faulty as I didn't have this problem before
The Onkyo receiver should report its own audio capabilities + the video capabilities of the TV connected to the receiver.
It is not the case, or partially the case. With a toolt called "monitor asset manager" I was able to see that no 3D formats were supported by the receiver, although the 3D formats were reported if I directly connected the PC to the 3D TV.
FIX THIS BUG ONKYO !
Meantime, you have to replace the faulty EDID (the information reported by the HDMI device).
Very simple with the tool :
1/ Connect the PC directly to the TV and launch the tool : under [Real-Time XXX] you will see your device and the 3D formats supported. Do "File" menu, "Create inf". Save it to monitor-tv.inf
2/ Unplug and then connect your PC to the receiver (also the TV should be connected to the receiver and running).
Do a refresh (F5), then File menu, create inf. Save it to monitor-receiver.inf (for backup)
3/ In other forums, it is said to merge the 2 files. I didn't, as only the video part is used by the system :
in hardware manager, right click on the monitor / generic plug and play monitor, update driver, search for a driver in computer, choose in the drivers list, then select the location of your modified monitor-tv.inf file, validate the unsigned driver
4/ Reboot and unplug/replug your HDMI cable. Try PowerDVD
If you have troubles do the merge :
1/ From file monitor-receiver.inf, search "[OVERRIDDEN-EDID_AddReg]" section, and copy the "HKR,EDID_OVERRIDE,"1",....." line
2/ In file monitor-tv.inf, replace the "HKR,EDID_OVERRIDE,"1",....." line by the one copied.