To explain in too much detail:
Philips moved their discovery endpoint recently, but this new endpoint does not work with v1 bridges. So if we use the new one we lose support for v1 bridges.
f.lux can also use uPNP on the LAN to find hue bridges, but this tends to be slower and more error prone (firewalls can break it - so this may be what's going on). This is what basically every f.lux install is doing right now.
We have been trying to keep support for v1 bridges but may give it up soon, or we may just make an option you turn on to use uPNP to support them--but breaking anything in an update is something to avoid.