I have exactly the same error, everything appears correct. If I got to https://office.mydomain.com/hosting/discovery I see the XML with a valid SSL certificate. Yet Like you I get the below error when trying to access from within NextCloud.
Collabora Online unknown error: Client error response [url] https://office.mydomain.com/hosting/discovery [status code] 404 [reason phrase] Not Found
Please contact the “https://office.mydomain.com:443” administrator.
Have you managed to get any further, I’ve tried all sorts for the last week and got nowhere. Do you think it points to the connector app, I tried completely uninstalling the Collabora NextCloud/OwnCloud app and checking the app folder was gone think it was called RichText or something similar then reinstalled but still made no difference.
Hoping you have got further than me or someone else can help us?