Problem with avermedia Volar Black HD (af9015) : recognised but not scanning

Hi,

I have an old avermedia Volar Black HD (af9015) that still works pretty well in windows 8 (it scans the whole dvb-t muxes in less than one minute and the image is perfect even in HD).
When I try it on linux, it takes 12 minutes to scan with w_scan, and despite showing lines such as :
updating transponder:
(QAM_64 f = 4294967 kHz I999B8C999D0T8G32Y0) 0x405A
to (QAM_64 f = 4294967 kHz I999B8C999D0T8G8Y0) 0x405A
undefined coderate HP
in the end, it says :

tune to: QAM_AUTO f = 482000 kHz I999B8C999D999T999G999Y999 (time: 12:13) ----------no signal----------
tune to: QAM_AUTO f = 482000 kHz I999B8C999D999T999G999Y999 (no signal) (time: 12:14) ----------no signal----------
tune to: QAM_64 f = 4294967 kHz I999B8C999D0T8G8Y0
(time: 12:16) skipped: (freq 4294967286 unsupported by driver) tune to: QAM_AUTO f = 482166 kHz I999B8C999D999T999G999Y999 (time: 12:16) ----------no signal----------
tune to: QAM_AUTO f = 482166 kHz I999B8C999D999T999G999Y999 (no signal) (time: 12:17) ----------no signal----------

ERROR: Sorry - i couldn't get any working frequency/transponder Nothing to scan!!

The problem is the same on my destop pc (debian 8, kernel 3.16.0-4-amd64) and on a Raspberry 1 (Linux osmc 4.2.3-3-osmc, or openelec).
I tried also with tvheadend, but scan does not work either.

The firmware is correct and installed in /lib/firmware. Dmesg shows that the usb device is well detected, with no errors :

[   13.846959] usb 1-5: dvb_usb_v2: found a 'AverMedia AVerTV Volar
Black HD (A850)' in cold state
[   13.847467] usb 1-5: firmware: direct-loading firmware
dvb-usb-af9015.fw
[   13.847474] usb 1-5: dvb_usb_v2: downloading firmware from file
'dvb-usb-af9015.fw'
[   13.917176] usb 1-5: dvb_usb_v2: found a 'AverMedia AVerTV Volar
Black HD (A850)' in warm state
[   14.327175] usb 1-5: dvb_usb_v2: will pass the complete MPEG2
transport stream to the software demuxer
[   14.335086] usb 1-5: DVB: registering adapter 0 frontend 0 (Afatech
AF9013)...
[   14.345704] usb 1-5: dvb_usb_v2: 'AverMedia AVerTV Volar Black HD
(A850)' successfully initialized and connected
[   14.345795] usbcore: registered new interface driver dvb_usb_af9015

And lsusb :
Bus 001 Device 003: ID 07ca:850a AVerMedia Technologies, Inc. AverTV Volar Black HD (A850)

I must say it is very frustrating to see a device still supported in windows 8, and working perfectly, but not working anymore in linux despite stated as supported in
http://www.linuxtv.org/wiki/index.php/AVerTV_Volar_Black_HD_%28A850%29

Thanks in advance for any idea that could help !
OD