I tried to add a smart proxy earlier, and was getting a silent do-nothing failure. It turns out that I had forgotten to enable the feature I wanted, and so it had no features - and foreman wouldn't add it, but there was no error message.

Reproducible:
Build a smart proxy
Configure SSL
Start it without enabling any features
curl the /features page, it should only say 'facts' or something like that
try to add it - clicking submit does nothing.