Are you using an external timebase source? What kind of frequency measurement method are you using (Low Frequency with 1 Counter, High Frequency with 2 Counters, Large Range with 2 Counters)? In traditional DAQ terms, this error means you received multiple edges on the GATE of the counter without receiving an edge on the SOURCE pin. How fast is the signal you are trying to measure? You can explicitly change the counter timebase source by settings two attributes: counter timebase source and counter timebase rate. These attributes are located in the channel attribute property node under
Counter Input->General Properties->Counter Timebase->Source
and
Counter Input->General Properties->Counter Timebase->Rate
I hope this helps.
gus....