It's not any try to exculpate ourselves by this issue, but i must say that Mental Ray always was a pain for us. Not for the render engine itself, but for the poorly implementation and (inexistent) support in 3DS Max. For developers, we must solve the problems by trial error. Almost there is not documentation of how it's integrated in Max, useful logs or debugging tools... nothing.
I don't know what percentage of Max users work with Mental Ray, but from our stats, most of our customers prefer other engines (specially V-Ray). Sincerely, if i had to rewrite now our plugins from scratch, i would considere seriously to exclude MR from the supported engines.
Said that, please see my previous post about the logs and test scenes. Any help that may help us to reproduce this problem will be welcome.