I have no idea. Digging into the sources tells me, that WebInputStream has no common implementation but is implemented in each platform differently, e.g. mac:
So I think createConnection failed (see line 615).
In line 406 you find the condition for URLConnectionState::start() to fail:
And if you follow that trail you end up deep in ObjectiveC code I don’t dare to interpret.
So maybe someone else can give you a more qualified answer…