Sonja-
  Sorry for the delay, but the latest releases of SPM12 do play nicely with the latest versions of Matlab (e.g. 2018a) and MacOS (10.13). Be aware that MacOS is not compatible with some older Matlab releases (e.g. 2012a). NiiStat should be agnostic across all of these. My guess is you have to reset the path of your Matlab. Make sure both SPM12 and NiiStat folders are on your Matlab path.