Acceleport Xp 16P not working

I use AcceleportXp 16P for communicating from my SCADA software to External Modbus Master. The modbus slave in my SCADA communicates through system ports COM1 and 2 but not through Digi ports. What could be the problem?
I used the following settings:
port_name = COM3
baud_rate = 19200
parity = even
stop_bit = 1
data_bits = 8
flow_control = none

Have you tried any of the other Digi com ports other than 3 or 4? It might be an issue similar to the following: