The first problem is that the 2ps rise/fall time in the model is bogus. The intent is to make a very fast edge as the stimulus and then run that through the S-parameter model that represents the buffer (and package?). Then the resulting filtered waveform should be representative of the signal out of the actual device.
HyperLynx handles this condition of the super-fast edge rate very well with the advanced complex pole fitting (CPF) algorithm. This is one of the strengths of HyperLynx. The warnings in your model assume that the simulator uses convolution to model the S-parameters, which are frequency domain data, in a time domain simulation. HyperLynx doesn't have these problems because it uses CPF to make a causal model of the S-parameters that cleanly extrapolates the data to infinite frequency.