MacOS Home Assistant SSL error after first connection

I have Home Assistant (all latest updates) running on my local network (10.1.1.2).
I can access it externally via my iPhone and iPad perfectly.
I recently installed the Home Assistant app on my iMac (again all latest updates) and I could set things up and access the server.
However when I exited the app and then opened it again I get an SSL error:


I’ve looked elsewhere and can see others wrtht he same issue but none of the suggestions either work or are not applicable (i.e. running windows etc.).
Why does it work the first time but not thereafter?
What settings do I need within my router, Home Assistand Server etc. to get this working.
(Please don’t say that I need to use the cloud - I’m a pensioner and I’m not paying for these external services.)
Susan