Vrms drops to zero - no more data

yes, it kept reporting, but just zeros:
The data is from an ESP8266
I have a USB->UART bridge, multimeter, etc

Log from emonESP:

2020-04-10T10:35:52.976Z MSG:150,Vrms:246.31,P1:-1501,E1:4294966701,pulse:0
2020-04-10T10:35:54.067Z MSG:153,Vrms:246.32,P1:-1500,E1:4294966689,pulse:0
2020-04-10T10:35:55.138Z MSG:154,Vrms:246.33,P1:-1510,E1:4294966685,pulse:0
2020-04-10T10:35:58.405Z MSG:158,Vrms:0.00,P1:0,E1:4294966685,pulse:0
2020-04-10T10:35:59.474Z MSG:161,Vrms:0.00,P1:0,E1:4294966685,pulse:0
2020-04-10T10:36:00.569Z MSG:164,Vrms:0.00,P1:0,E1:4294966685,pulse:0
2020-04-10T10:36:01.643Z MSG:166,Vrms:0.00,P1:0,E1:4294966685,pulse:0
2020-04-10T10:36:02.734Z MSG:167,Vrms:0.00,P1:0,E1:4294966685,pulse:0
2020-04-10T10:36:05.966Z MSG:170,Vrms:0.00,P1:0,E1:4294966685,pulse:0


Pressed reset:

|2020-04-10T10:40:49.414Z |i4Cal = 16.67|
|2020-04-10T10:40:50.024Z |’+++’ then [Enter] now to enter config mode…5s|
|2020-04-10T10:40:55.373Z |CT1 detected, i1Cal:90.90|
|2020-04-10T10:41:05.450Z |AC present|
|2020-04-10T10:41:06.077Z |MSG:1,Vrms:244.94,P1:-1360,E1:4294967293,pulse:0|
|2020-04-10T10:41:15.913Z |MSG:2,Vrms:244.87,P1:-1354,E1:4294967289,pulse:0|

(-ve as exporting from solar PV - issue occurs when +ve as well)