Disconnect entering Island Expedition

I’ve reached the War Campaign quest for Island Expedition, but I’m disconnected with the WOW51900319 error at the end of loading. Searching for that error suggests real connection issues that I do not have. I have no other connection issues with the game. I’ve tried numerous times, always with the disconnection. Occasionally I’ll catch a glimpse of the game before being booted.

I have already Scan & Repaired and deleted my Cache/Interface/WTF folders. I do not run add-ons. I seem to remember a similar issue in the past, entering scenarios, but that was usually solvable by simply reconnecting and trying again. No luck with this Island Expedition quest.

If for some reason this matters, I am on a Late 2013 iMac running macOS 10.14.6.