Most likely you may have loaded a bad program into the PLC and is causing communication problem.
What you can try to do is to turn DIP switch #4 to ON and reset the PLC (power off and ON). The PLC should start up with the "PAUSE" light ON and it means it is not running the user program. Now try to use TLServer to communicate with the PLC again. If it works then overwrite whatever program inside the PLC with a blank program. You would then have recovered the PLC from error condition.
If this still does not work, then it could be a bad DB9 cable, a bad USB-RS232 connector (What brand of USB-RS232 converter are you using?), or a bad RS232 chip (ST232 or MAX232) on the PLC.
Rgds