I experienced difficulty connecting to VarAC users running SDR's on two occasions while testing version 3. In both cases, repeated attempts to connect to these users failed, but they could connect to me without difficulty. I also use an SDR.
I was suspicious, since I have never experienced the "latency connection issue" described in the VARA-HF "manual". I used latency monitor, https://www.resplendence.com/downloads, a nice software tool for identifying the cause(s) of latency. Running latency monitor identified an issue with ndis.sys, which is a network driver issue. Apparently, a recent Windows 10 update updated something.
I won't go through all of the "detective" work. I isolated the issue to the Intel network adapter. Searching the Intel driver site, I found an updated driver for the ethernet adapter. This driver was NOT identified by the Windows update process, or by the Dell driver update process. I downloaded the driver directly from Intel and installed it--latency issue solved.
Latency monitor is a good tool for identifying any issues that might cause issues with "real-time" pc audio.