Lost USB comms after a killpower - BX700U

[BillP]

I have recently purchased a BX700U and am using it with apcupsd on three small servers and a  router, where the servers run Ubuntu 16.04 and Debian Stretch.

I have been taking my setup through test cycles as I understand the operation of the BX700U and the apcupsd software for my first time.

The following has happened to me twice now, and would be ridiculous if occurs regularly as I envisage unattended use...

(a) I remove the power to the UPS and let the Master and two Slave work towards their shutdowns....
(b) The Master issues a apcupsd --killpower followed by a shutdown -h now
(c) as expected the Master shuts down and moments later, the UPS powers off.
(d) the total power draw is about 25%, and I am testing with a TIMEOUappvalley T 240 parameter, so the battery is not being discharged by anything significant.
(e) When the Power to the UPS is restored, all devices powetutuappr up as expected.
(f) HOWEVER there is no USB comms to the UPS.
(g) the fix (so far twice) is to power off the UPS and remove the Yellow plug, and leave overnight.
(h) Clearly ridiculous. Is this a known problem? Do I have a faulty unit?
(i) This is NOT a problem with the USB cable, the USB port or the Server itself. All have been checked by substitutions, including the Powershute software on Windows 10. (g) is currently my only fix.

UPS manufactured 3rd Dec 2017
Software version : FIRMWARE : 924.Z3 .I USB FW:Z3

Advice please
Thank you