I am having this same issue.
I have the Credits module installed as well.
If a member goes to https://domain.com.au/page/acl-view and if I only have "PAYPAL
PayPal payment provider" active (not the API) then the option to buy with paypal does not appear and it forces users to by Credits (with paypal) and that work-around works.
If I try activate the API (which works fine in Sandbox mode) it fails in Live mode with that same error as above.