While troubleshooting a template manager bug found in Pro using Chrome in OS X (that discussion is here: https://theme.co/apex/forum/t/template-manager-unusable/27334/2?u=simeoned), I launched Safari to see if the issue was present there. In doing so I found an even more significant problem:
Pro won’t launch at all. Headers, Content, Footers, Templates, Global Blocks, Colors, Fonts and Theme Options, Nothing will get past the spinning circle. I’ve waited a long time to see if it was just reeeeealy slow. But no. It just doesn’t launch at all.