I haven’t been able to solve this problem yet, so I’m throwing it out to the world. I’d throw it out to Apple Support forums, but they’re apparently doing something to them (and have been for the last few days) and they won’t let me post – I get a backend-under-renovation notice whenever I try to log in.
100% of the time, I can reproduce this error when I try to edit the timestamp of a comment in WordPress. It happens literally every second time I try to make such an edit:
It also happens when saving or editing posts. Not every time, but enough that I’ve started saving posts in the paste buffer so I don’t lose them. It will also reproduce on other WordPress administrative pages involving post or comment edits – commits, really – enough that it will happen during most sessions. If you search, you’ll find other people are experiencing this as well; it’s been going on since at least Mavericks.
It does not reproduce in Firefox on the same machine, at the same time, under any circumstances. There have been multiple posts to Apple support communities about it, but none of the solutions offered – solutions which did fix it for those posters – fix it here. That includes deleting all saved website data, disabling all (non-Apple) extensions, disabling all plug-ins, deleting add-on input methods and scripting additions (of which I had one of each), everything here.
But the one thing I have found is that it does not reproduce in safe mode. At all. Something specific is causing this.
Looking for the Grammy Awards Long List nominee post? Thank you for listening, and for your consideration.