Error -200685 DAQmx Write Counter Frequency

I have to generate 4 finite pulse trains with Counter Output and a X-series board (NI PCIe-6321). I know that with X-series boards only one counter is used to generate finite pulse train. I use LabVIEW 2010 sp1 and DAQmx 9.3.5.

One task for each counter is created.

When only one task runs, everything is ok, but, when more than one task runs, i receive "error -200685: Pulse frequency specified is not supported for this device given the Counter Timebase Rate." from DAQmx Write Counter Frequency. Error reports invalid data: 0,000000

I have checked data supplied and no zero frequency is passed to the VI. In fact, counter frequencies lower than 100 are forced to be 100 through a previous VI.

Thanks for the help