STM32 F4 Discovery: Problems with PWM

Hello,
I have some problems with the following Code from this example on the STM32 F4 Discovery board:

function setServo(pin,pos) {
 if (pos<0) pos=0;
 if (pos>1) pos=1;
 analogWrite(pin, (1+pos) / 50.0, {freq:20});
}

setServo(<PIN>, 0.5);

On some pins (I've tested C9, A8, B13, E13, E14) I get half the frequency I have selected (measured 10 hz with DSO). On these pins (C7, C6, C8, B14) I get the expected results.
Wrong initialized timer?