This error message appears when at least one object in the scene has two or more modifiers of the same type applied to it:
This is currently an unsupported scenario. The rendering will continue, but only one of the modifiers of the same type in the stack will be used.
Select the problematic object (there is a "Select object" option for this in the error message window) and remove all but one repeated modifiers from its stack.
For example, if the object has two or more CoronaDisplacementMod modifiers applied to it, remove all but one of them: