Is there a way to Reindex/"Reinstall" AirPlay Devices in OS X?
Typically when I see issues with AirPlay, particularly when you've taken the troubleshooting steps that you have already, it is a network issue and not a software issue.

So here are a few things to try/verify:


Can you AirPlay from this Mac to other AirPlay receivers?
Are you on the same physical network as the AirFoil receiver (including same Wi-Fi network—and the same frequency band (2.4 GHz vs. 5 GHz)?
Is Bonjour working? 
Are all multicast filtering options disabled on your switch/router (or conversely, is multicast traffic permitted)? Note that if your router autoupdates, a setting or default may have changed.
Have you tried temporarily disabling all firewalls on both machines?