Okay, my mistake.

My source file has 9 columns. The target file has 6 columns excluding Date. I had forgotten a tab to filter out columns of the source file (in this case <VOL>).

Code:
//<DATE> <TIME> <OPEN> <HIGH> <LOW> <CLOSE> <TICKVOL> <VOL> <SPREAD>
// STK line format "2018.01.01 23:00:00 1.20098 1.20103 1.20086 1.20086 12 0 21"
string Format = "%Y.%m.%d %H:%M:%S f3 f1 f2 f4 f6  < 2Tabs > f5";