Oddly, each time this has happened (twice, around 24 hours apart), it seems to go away itself after some hours. Which suggests it might be something that's cached, as caching usually self-clears in a few hours.
The latest incident cleared itself.
If it happens again, I'll manually clear that cache as you suggested, and see what happens.
What concerns me is that this is even occurring at all. Why would Wordpress homepage suddenly start redirecting to
https:/// (three forward slashes, which to me suggests there's a redirect arising from somewhere, and that redirect is missing what should be between the second and last forward slash (e.g., https://mydomain.com/