Warning: wrong connectionId in received user packet

Hello,

I am running into weird random issue with Unet client-server during handshake communication.

Sequence:
1. server is ready and running
2. client is starting and connecting to server
3a. (very positive scenario) connection is setup immediately
3b. server is throwing "Warning: wrong connectionId in received user packet"
3b1. (positive scenario) after some of these warnings the connection is setup and everything is ok
3b2. (negative scenario) connection is not setup

Unity...

Warning: wrong connectionId in received user packet