The problem (natron up, safari cannot load page) happens here as well. I tried to change the settings as per you suggestion and it did not help. For me, it is a show stopper!
P.S. Chrome works well when Narton is up, so it is some kind of interaction specifically with Safari.
I am running on iMac with OSX High Sierra
It gets even more interesting. If you close Natron, starts Safari developer window (develop/show web inspector) and then start Natron, Safari does load pages, however if you close safari's developer window while natron is up, it will stop loading pages.