Due to a problem in the Quartus® II release 14.0 software, this error will occur when a LUT has an output type of "Inherit via Internal Rule" but not all the internal vector elements need the same output type.
The data for a LUT is a matrix, and DSP Builder decomposes the LUT into a LUT for each vector element.
With the decomposition of the LUT, most of the decomposed LUTs might have the same output type (for example ufix23). But if one or more of the elements might use a different output type (perhaps ufix22). This data type is inferred from the data in the LUT, which could be different for each component.
DSPB will then try to combine these outputs back into a vector, and fail to infer its type, because not all of the components in the vector have the same type.
As a workaround, try explicitly giving the LUTs a specific output type rather than setting them to "Inherit via Internal Rule".
This will be fixed in a future Quartus II software release.