Bug (Case 1383330) Unity Transport shows warnings when connecting from remote endpoint.

Thanks for the report!

This is something we're looking into. Other users have also mentioned seeing the same behavior. The warning (which I concede is not very clear) is normally printed when we can't send our full send queue to the OS in one single update. Typically that means some packets may only be sent on the next update (usually the next frame). Now there's something fishy going on when sending to remote endpoints on Windows that causes this to happen very often, even for very little traffic. It's yet unknown if this is something we're doing wrong or some quirk of Windows.

The takeaway is that if you don't observe any traffic loss (you can check with Wireshark that everything actually makes it to the wire), the warning can be safely ignored. But I understand it sucks to have the warning spammed all the time, which is why we are looking into this.