Thanks for the response. I just did a quick check, and under "Server" -> "Signed into" it shows "https://...", so it appears to be correct (unless it's not really communicating over https?). I should also mention I'm publishing from Tableau Desktop 2018.2.0.
In addition, I've already published other data sources in the same session logged into Tableau Online successfully, for some reason it's just stopped.
Thanks for your time,
I'm assuming your Tableau Online version is also 2018.2. If it is, then you may have to contact Tableau support team to have a look.