Yeah, I think your priors are not informative enough, and I suspect that’s one reason why sampling is slow when cores=1. But if this were the cause of the error, you should get an error message when cores>1.
This looks a lot like a multiprocessing break issue 
@lucianopaz, I know multiprocessing breaks on Windows and we’ve had some examples with recent versions of Python and MacOS, but is it also the case on Linux now? 