@admin fwiw, it doesn't happen all the time. And as soon as I hit the time-mark, all is fine.

It tends to happen when I first open up Mastodon. If I stay on long enough, it happens every 15 to 30 minutes. I can't post a toot while it's happening, though I can scroll the feeds.

Another detail, fwiw, is that these notices don't pop up just once - the pop up and stack in a ever-climbing pile, then go down in number, then back up. It's visually weird but interesting.