In order to spare any future comers a similarly tortuous journey: I solved the issue of the missing Netscape extension by adding
set_var EASYRSA_NS_SUPPORT yes
to easy-rsa’s “vars” file before generating the server certficiate - which now contains
X509v3 Extended Key Usage:
TLS Web Server Authentication
X509v3 Key Usage:
Digital Signature, Key Encipherment
Netscape Comment:
Easy-RSA Generated Certificate
Netscape Cert Type:
SSL Server
and thereby satisfies the RV50’s retrograde certificate verification. But really Sierra, get with the ******* programme already. Netscape gone dead long time, and most of their proprietary crap went with them.