Hi Eoin,
I am having the exact same problem. Was working up to a few days ago, then not. MS have said this is their problem in the past. I've had to resort to using other tools at my expense in the interim. I get the exact same message when I try the Read Aloud function. I'm told to log in, even though I'm already logged it. I cannot log out, as I like the 'always on' features of being on Cloud. MS just need to sort this issue.
It is apparently a bug that is for all devices for all Office applications.
Writing here does not bring it to the attention of anyone who can do anything about it. You are addressing your fellow users here. Microsoft is aware of the problem, though. They are scrambling to fix it.
I recommend using the Public Feedback Portal.
https://feedbackportal.microsoft.com/feedback/forum/fb6d67e3-301c-ec11-b6e7-0022481f8472
Then, once your feedback is posted, copy the URL and then come back here and post it so that others can comment and vote.
Be the squeaky wheel.
You can search for and vote on other feedback already posted. I would suggest a search for "read aloud."
Here is a list of some feedback links:
Feel free to vote and vent a little.
For more on using feedback, see my article: This is a user-to-user support forum. -- You can give feedback TO MICROSOFT designers and decision makers on Microsoft Word or other Microsoft Products using their feedback mechanisms, (not here). (This is a link to directions.)