I was having the same issue. The root of it was that at one point i had logged in and tried Nabu Casa, but then switched to using my own reverse proxy. When logging in from the companion app, it sees your nabu casa login and still tries to connect to it, but since my subscription expired, it just gives SSL errors on the nabu casa url.
To fix:
Log in to your HA web admin portal
Go to Configuration > Home Assistant Cloud
Click the three dots in the top right
Click “Sign out”
Then try to log in to the companion app on iOS again and it should work.
I got the same problem today. I could not login via the ios app. The solution above failed. I was already logged out from the cloud.
Instead I opened the config in the app and checked the instance where I login. I said I’m logging in via the cloud. When I disabled this it started to work again.