TPUMP Error 3604 "Cannot place a null value in a NOT NULL field."

[SirThinksALot]

I randomly in processing data in TPUMP receive an error code 3604 (with error message "Cannot place a null value in a NOT NULL field." This occurs about 1 in 1000+ records processed for UPDATE statements (which involve a join). The data is correct (field value used for update is non null valid timestamp(3)) value. I have checked it and also a subsequent insert in the tpump script with the same data is successful.

Any ideas / similar experiences?
I am using tpump 1.05.00 on V2R4
My next step is to experiment with tpump 1.07