I've seen that the Volume Regions attenuation calculations in some cases are incorrect, generating wrong colors, sometimes even negative colors.
For example, wrong calculation:
I've made a change that I hope solves the problem. The change, which will be included in the next YafaRay version, is here:
With the change applied, this is the correct render:
I will upload the scene in the first comment after this entry. For some reason the YafaRay website does not work well with attachments in the first post of an Issue entry.