Modbus TCP Connection to Remote Virtual Server

I try to install a Modbus connection to read data from an instrument to a Data acquisition system located on a remote virtual server (VMware) using a software Datafeed Modbus OPC server, but it fails. After about 30 seconds "No connection to PLC". Ping is ok, telnet to the TCP port (502) is ok (some delay at the first call).

Some Modbus test clients (GenericModbus/Jbus Tester, RMMS) are connecting and transferring data. Another PLC on the same switch is working without problems. I have no idea what can be wrong with this...