Since it is possible that this internal error resulted from an unhandled usage error in your design, we advise you to carefully check the usage of the block reporting the internal error. If errors persist, we recommend that you restart MATLAB.
The possible cause of internal/fatal error is tool itself. Are you using compatible sysgen and Matlab version? Do you meet the error with sysgen example design? If all the demos fails, it must be due to tools itself, one possible way is to reinstall tools.