Well, I'm a bit puzzled here.
I realised increasing these 2 resistors value was in fact also increasing the voltage between the 2 transistors emitters :
with 2 X 12K -> 16.48V between the 2 emitters
with 2X 10K -> 15.12V
with 2X 8K2 -> 13.12V
so I went back to 2X 8K2 and it is now working fine !
But I didn't change the 2X 4K7 from the voltage divider, so I now have an hybrid version in regards to the values you recommend on the module page, but it works
Thanks
ph