Thanks so much for persisting Dave!
A good tip from your colleague, but a thorough review of the DomDir revealed no issues with the existing Internet Site documents.
I am certainly accessing the exact same server/database with both commands - the only thing different is the protocol.
Funnily enough, web services that I have developed/exposed in the same database (simple Lotusscript agents that return JSON data) work fine when called via HTTPS.
So, this URL works fine...
https://xxx.abc.com.au/web/data.nsf/v1/doThisThing
... but this one does not...
https://xxx.abc.com.au/web/data.nsf/api/data/collections/name/DocsByDate?start=0&count=10
I've also just noticed something that may be a contributing factor here.
When I attempt to access the host root, I ALSO observe a 403 error. So going to https://xxx.abc.com.au, I observe...
Http Status Code: 403
Reason: Unable to process request, directory browsing is not allowed
This is getting even stranger!
It's not that I even want to access the root, but why on Earth would it reply with a 403 error?
The 'Internet Site' document lists the 'default home page' as 'index.html' (a flat HTML page) and going to https://xxx.abc.com.au/index.html works just fine.
I really need to sort this out and will continue my investigations/trials.
I'll let y'all know if I manage to get to the source of the issue.
Cheers!