This is the mystery.
Here, 5 days ago we get an answer from someone:
I’m sorry to hear that you’ve all encountered this issue. We’re aware of these reports and we’re looking into the problem. It would appear that this is, for the most part, fixed after scheduled server restarts. Unfortunately, we still don’t know what’s causing it, so we don’t yet have a fix. We appreciate the reports and will continue investigating this for a possible solution in a future update.
They don’t know. So the current plan is to report your server is down and wait 12 hours.