I’m going to put this here for information. I had reinstalled using hassOS, and got a new (I thought) cert from letsencrypt.t It was good through December (supposedly).
Today my iPhone app for HA would not load. Investigation showed this was due to an expired (!) certificate. The cert, when examined in Firefox, showed an expiration of 1 November.
So I started out trying to remedy this.
Starting the LetsEncrypt add-on gave me “certificate not due for renewal”, since it thought it was still good. (Why it thought that, I have no idea. The supervisor logs in HA also showed cert expired.)
So I uninstalled/reinstalled LetsEncrypt. Install successful, cert now expires 04-12-2019, as expected (180+days? Not what LetsEncrypt claims, but moving on.). But iPhone and Firefox both say “expired certificate”.
The solution (I hope it sticks) was to uninstall LetsEncrypt, do a host reboot, then reinstall LetsEncrypt. All well now.