It looks like the change to the Network module this past November might have done something funky on Linux, I updated to 4.3.1 recently, and previously-working code started failing.
I’ve been running a local Virtual Machine running a Flask server that implements a simple API. Since it’s local my URLs have looked like http://localhost:9090/<rest of API call> and the last version of JUCE I was on (4.2.4) handled everything. After updating, every request to the localhost times out.
What sticks out to me is the response codes from the server: the call is made, and the server responds, but all of this happens after the timeout length. If I set a breakpoint immediately after the URL::readEntireTextStream() call, the program operates, waits for the entire timeout length during that call, after which the breakpoint (with the returned empty string) and 200 response from the server occur as near as makes no difference to simultaneously.
I have not taken the time right now to dig deeply, I was trying to fix an unrelated bug so I rolled back to unblock myself, but I will revisit this when the bug I authored does not consume my attention.

