Вот ответ из WRC:

I think the error is exactly what is says:
ERROR #6059: Unable to open TCP/IP socket to server localhost:80

I have tested for more than two hours and got similar errors, but sporadically - about 20 in this
period.

Not sure yet but it looks that webserver is not able to open new connection for incoming request.
The error comes from the OPEN command in %Net.HttpRequest.Open() method:
Open "|TCP|"_dev:(server:port:"SCWD"::8192:8192:/TCPNOXY):$zobjval(,/*Timeout*/26,0,3,26) Set
sucess=$T

I will investigate it later, it looks like somehow related to custom error trapping in AddInteger
client method because I don't get this error when testing non change DemoProxy class.

Continue tomorrow.

Установил твик который меняет tcpip.sys и, соответственно, увеличивает доступное количество tcp соединений. Ошибка 6059 выскочила в первые 30 минут работы