I have problem with YafaRay engine too:
when I start rendering, a lot of text is logged into the console and then nothing happens - nothing is rendered.
But sometimes it works without problems.
I think it gets stuck before it has log this

[16:33:14] INFO (+ 1s): Environment: View number = 0, view name: '', camera name: '5b3b3366-0813-49fd-b31a-250e4ba96d35'
[16:33:14] PARM: Using [4] Threads.
[16:33:14] PARM: Using for Photon Mapping [4] Threads.
[16:33:14] INFO: Kd-Tree: Starting build (7945647 prims, cr: 0.8 eb: 0.33)
[16:33:42] INFO (+ 28s): Scene: total s ....

With engine Sunflow is rendering working perfect, but with YafaRay sometimes not start rendering, sometimes java crash.
I use linux and 64b java with -Xmx4g.
And I have to admit that my file is big (180MB), but Sunflow has no problem with that.

PS: I am sorry for my english