BuddyPress.org

I figured out the problem was because I was using the file based caching that the BP developers recommended here: https://codex.buddypress.org/getting-started/improving-performance/. They might want to rethink recommending something that creates such a big problem! Unless you are going to turn off private messaging, you can’t use that. I’ve gone back to WP-Super Cache (with caching turned off for logged in users) and that seems to work OK.