Bizarrely, the answer is “fonts”!
Spent ages last night trying to work out why on my new PC forwarding 9418 to localhost port 9418 wasn’t working:
Curiously, X11 forwarding worked, and plink -L 9418:localhost:9418 … worked too – but the above didn’t. Can you see it?
The new PC has lots of pixels and the text was small. I first typed this
but it looked to me like I’d missed the colon, so I added one – see above. Turns out localhost::9418 is not a valid port forwarding destination, but there’s no error. It just silently failed.
Changing it to localhost:9418 (as I originally had it) of course made it work. PEBKAC again 🙁 .