Synchronize Reference Clock error

Hi,

I am using 2 synchronized PCI (thet are of different models) with a RTSI cable and I am acquiring signals from both. In signal Express they work fine (and also the automatically generated code does) but in the examples such as "Analog Input Synchronizatio.vi" they do not.

Particularly, the error I get is the -200452, regarding the RefClk.Src of DAQmx Timing but whatever I use as Reference Source Clock nothing changes.
I am completely new to Labview: can you explain how to solve it and why I get this?

Thanks in advance