thank you very much for the speedy reply.
I had kinda deducted from the related threads that those where the files in question and did find them in the respective directories. Hence I was all the more clueless. I found the solution yesterday though, albeit a weird one.
Turns out puppet_shaders, which I had installed the latest versions of recently (beta), apparently directly or indirectly were the culprit. p_mix was creating an an error on Max start-up which I disregarded since I don't use that. But that got me thinking that the error might interrupt a loading sequence, kinda like in Maya's Mel , when a script stops executing once it encounters an error. Anyways, whether or not that is the case I have no idea, but once I quoted the lines related to loading puppet_shaders out in the respective .mi file in the mental ray folder and restarted Max, Mental Ray once again rendered the Forest instances without a problem.
I'll have to roll back to an older version of puppet_shaders, it seems.
Can you provide any more insight on this from your understanding of how Max/Mental Ray work? I'm not as experienced with Max as I am with Maya, that's why I'm asking.