I’d like to say I’m having the exact same problem as the original poster and I’m also on the same ISP as him (UPC Ireland). I’ve attached two log files. In both cases I start the launcher (in debug mode), fail the first login and succeed the second (as always), go to Library, click the button to install the engine and immediately get the error message listed at the top of this thread. Try once or twice more to download the engine with the same result, and then just close the program.
It’s obviously a frustrating issue and prevents me from using the Unreal engine.