Assuming that one of these was the cause, I activated each in turn and reloaded. None of them made any difference, except a single tracker from JanRain. Accepting this tracker let everything work perfectly. Reading more about JanRain though -- and particularly its interaction with Adobe analytics (which it also tries to load) -- I discovered that they wanted to "create a holistic view of your business by collecting, analyzing and reporting all customer interactions. To derive the most actionable insights, you must link your customers' actions with who they are and what their interests are. Janrain bridges the gap by connecting demographic and psychographic data, collected through traditional and social login, with Adobe's behavioral data, so you understand the whole customer journey".
I do not want them to do any of this, and don't think I should have to. Interactions with Slate's 'support' were excruciating and -- while they at least didn't ask me to restart my computer -- they actually ended up saying that allowing these trackers is tied to their login process and I have to either accept or get a refund.
Robot 5x asks: Is it unacceptable to have to accept being tracked as a paying customer for new sites? "Or am I just being a big baby?"