I have access to several Salesforce Professional accounts with the API Access addon purchased. I also have a Connected App which I've created which I've oauth'd with Users with System Administrator privileges from these accounts.
With one of the accounts, this works perfectly. I can pull data via the REST API with no issues.
However, with some of the other accounts, I'm still receiving the error:
API_DISABLED_FOR_ORG: The REST API is not enabled for this Organization.
When trying to access the API.
From poking around inside all the accounts, I can't find any difference between them -- all the users who OAuth'd have Admin privileges, which should allow them to use the API.
Any idea what could be going on here?