If you see something like this:

[Arnold]: MAXtoA_Shaders.dll was compiled against non-compatible Arnold 5.0.2.1

it usually means you have an older Arnold trying to load something compiled with  a newer Arnold. There’s two common reasons for this happening:

• The system PATH includes the location of the older Arnold, so the older Arnold is loaded instead of the Arnold version included with the plugin.
• The plugin install is messed up, and has the wrong Arnold version in it.