Today, I tried to perform multilevel path-analyses using a SPSS-datafile saved as a .dat-file. Running the Mplus-imput file resulted in the next error: "One or more between-level variables have variation within a cluster for one or more clusters". However, inspecting the data-file made clear there are now differences within clusters.
Your data could be being read incorrectly for a variety of reasons -- blanks in the data set, commas in the data set instead of periods for decimals etc. Please send your input, data, output, and license number to firstname.lastname@example.org if you can't figure this out.