Olivier,
The problem has to do with the fact that you have double-buffering enabled when you call SCXI_Strain_Null. This will cause the -10636 (You cannot start a continuous (double-buffered) operation with a synchronous function call) error. The solution to this problem is to disable double-buffering before calling SCXI_Strain_Null. The function DAQ_DB_Config allows you to disable and enable double-buffered mode.
Good Luck. . .
Spencer S.