Hello @James Coates , Thanks for using Microsoft Q&A Platform.
Sorry for the inconveniences that has caused. Can you confirm if you haven't made any changes to the subscription; like created one in less than 24 hours something like that?
Generally, the issue might be related to the API endpoint or subscription key. Looks like you have done troubleshooting steps in double-checking that you are using the correct regional API endpoint for your resource in the document intelligence studio.
If you are sure that you are using the correct API endpoint, you can try re-generating your key in the Azure portal (under your resource->keys & endpoint) and using the new key to authenticate. It is possible that the key might expired.
If neither of these solutions work, I would suggest raising a support ticket in the Azure Portal for further assistance. I hope this helps.
Regards,
Vasavi