Running btest server on ROS v6.38 (current, stable) and btest.exe v0.1 on Win10 (64-bit), wirelessly between Win client and ROS AP, causes ROS log entry "login failure for user admin via bandwidth-test".
The admin account exists and works for everything else (winbox/http/etc).
On the btest.exe client (also run as administrator) side it shows "running..." for a ~two seconds and then "disconnected".
Entering the wrong credentials causes "auth failed" and disabling authentication doesn't change issue from "running..." for a ~two seconds and then "disconnected".
No firewall rules, so port UDP port 2000 (default) not blocked, antivirus/firewall disabled on Win, and even if it did the error message would be wrong, especially since ROS is able to log the event.

http://www.mikrotik.com/download/btest.exe

Client works to public btest servers http://forum.mikrotik.com/viewtopic.php?f=2&t=104266 (however only if using direction "both", not "receive" or "send", strange)