If you have specified all of the following:

1) the DIST=MULT option in the MODEL statement,
2) a CLASS statement with option PARAM= set to anything other than GLM,
3) a REPEATED statement,

and if the data contain missing values for one or more of the CLASS variables, then PROC GENMOD may abnormally terminate issuing a Read Access Violation, Segmentation Violation, or other error depending on your operating system.

To avoid the error, either remove all observations that have missing values for any of the CLASS variables, or specify the nonfull-rank parameterization PARAM=GLM.