I guess this hasn’t affected many users as it appears to be just me (I googled enough to safely assume that), but even so I’d consider reverting this.
My issue was an absolute nightmare to diagnose, I was using an additional include conf (domain.nginx.ssl.conf_extra) along with the default nginx template, and it results in the most non-descript error for the client and absolutely nothing is logged on the server.
Annoyingly I scoured through just about everything else before finally spotting the problem in the template.
I’ve now made a copy of the default template and removed the troublesome line.