TCP/IP commmunication between two target PCs

Hello,

I am trying to let two target PCs communicate to each other using TCP/IP block.

I used the example (link), but simplified it. Particularly, in the 2nd PC, the received message from 1st PC is sent to 1st PC immmediately without additional processing. However, the performance is TCP is not stable. After sending few packets, the 2nd PC shows the error "TCP Send error: EINPROGRESS". Please see the attached screenshots for more details (including target PC1 target PC2).

Could anyone help me to figure it out? Is the TCP/IP block unstable by itself?