Hi, there could be few reasons for this error, for example I simulated a PAY with Adaptive Payments and I received an error 520002 when I didn't pass in the header request all the 3 or also 1 missing of the following variables (the values below are just examples):
 => X-PAYPAL-SECURITY-PASSWORD: AAAAAAAAA88AA88A is the api_password
 => X-PAYPAL-APPLICATION-ID: APP-80A0000000000 is the app_id
 => X-PAYPAL-SECURITY-SIGNATURE: AA88aA.nA88A888aa8AA888aaaaaaaAA is the api_signature
But, if you still receive this error in these days, would be helpful to see your full request and the "correlation ID" returned in the response to check into the logs.