No transmission with 120SR/W5100

I operate a 120SR with VSP 2.0.2. Both UARTs show up in Win10 as Com6 and Com7, handshake is off in WIZ120SR_CFG.exe and VSP.exe as well
Cross-connecting both ports allows operation of my test program, written in .NE,T working fine, transfering data from Com6 to Com7 and vice versa.

When I connect a device with a RS232 interface to one of the 120SR-Ports (equipped with MAX202 as V24-level shifters, only RxD and TxD are used) and start the control-program for the device, transmission of data to the device is blocked completely. (Port Read and Write access are set to “All”)
Watching TxD at the output line of 120SR with an oscilloscope shows no activity at all.

(I have checked the device along with the control program with a USB/SerPort-converter - everything is working fine.
Same with WIZ 550S2E and VSP)

When I switch from RAW to Telnet-comm. in VSP I do see data on TxD of 120SR triggered by the device control program - but content does not fit to the pattern sent via USB/SerPort-converter so the device cannot be operated via 120SR.

Does anybody have a glue what´s going on here?
Any help is appreciated.