Thank you cattledog!!!
That seemed to fix my problem!!!
I now get 15kHz PWM frequency on both boards! at OCR1A = 512; I get an avarage voltage of 2.5V 
I'm so happy now!
resetting TCCR1A/B ( =0 ) first, then setting the right bits...
So the setting/resetting of those registers is just initially random then :o
So there shouldn't be any difference on the initial setting between UNO and MINI, I just have to have control on the bit setting, by resetting the whole register first!!
Thank you so much for your help!
regards
HanFre