Probably the Mental Ray shader and is not the same version that the plugin. As instance, the shader of Forest 3.0 doesn't work with the plugin Forest 3.01, giving this error.
The plugin file is:
<maxdir>\plugins\ForestPackPro.dlo
and the shader files are:
<maxdir>\mentalray\shaders_autoload\include\FShader3.mi
<maxdir>\mentalray\shaders_autoload\shaders\FShader3.dll
In the latest releases (3.03 if i remember well), we included a check to show a message if the versions don't match. To fix the error, reinstall the software or copy the right files from other node.