OK, on my devices (10.2R3.10), J-web works on my loopback interface without having to specify the loopback interface in the system -> services -> web-management -> https context.
I set a few options to look more like your config for testing:
system {
services {
web-management {
traceoptions {
file jweb-trace;
flag all;
}
management-url my-jweb;
https {
system-generated-certificate;
interface ge-0/0/0.420;
}
session {
session-limit 4;
}
}
}
}
I can connect to J-web using the device loopback address.
I assume you're trying to connect to J-Web from inside the Trust zone?
You could try setting a traceoptions file and see if anything is logged. You could also set a flow trace to see if the traffic is being dropped for some reason.
I see you've got IPv6 configured on this device, are you trying to access J-Web via IPv6? I'm not sure if having IPv6 turned up would have any effect on your traffic, as I haven't set up any SRX devices with IPv6.