I updated to JetPack version 4.0 this morning and soon after found that two of my sites threw a 500 error on the public side – only Chrome told me it was a 500 error, Firefox and Edge just gave the WSOD with no explanation. The page source was totally empty when the sites were viewed with all 3 browsers. I was still able to access the admin side of both sites.
Deactivating JetPack resuscitated the sites, Interestingly, three other sites with version 4.0 of JetPack are okay (so far, anyway). I suspect interaction with some plugin that is not common across all my sites, but with a number of people already experiencing the issue I’m just going to leave JetPack deactivated for now and wait for a fix.
This is the thread on the JetPack support forum about the issue: