Which Forest Pack Pro version are you using? The latest Forest Pack Lite/Pro 4.4.1
version has some mental ray® bugs fixed. Also the Forest Pack Pro 5.0.3 beta
comes with another mental ray® bugs fixed. I would recommend you to try those, if you're using an older release.
Anyway, would you please save and send us the minidump
file of the crash / mental ray log file? Thanks in advance.
Also, please feel free to send us the simplified scene (containing problematic Forest object(s)) through WeTransfer
to firstname.lastname@example.org to analyse it here.