I have inherited a .NET PayPal implementation for our corporate website. I have a sandbox account utitlising API userbame/password/signature with an endpoint of https://api-3t.sandbox.paypal.com/nvp
Once the order details are passed to PayPal I see the url of sandbox.paypal.com and can log via my "buyer" sandbox account (which has funds). I select to pay the order but then if successful it should go to my "Success" page , but instead I just get this: https://www.sandbox.paypal.com/checkoutweb/genericError?code=U0RLX0lOVEVHUkFUSU9OX0xPQURJTkdfRVJST1I (and details of the cancel url). I noticed that U0RLX0lOVEVHUkFUSU9OX0xPQURJTkdfRVJST1I decodes back to SDK_INTEGRATION_LOADING_ERROR but can't find any details on what this might mean.
We reference these 2 .NET dlls:
PayPalCoreSDK.dll ver 188.8.131.52
PayPalMerchantSDK ver 184.108.40.206
.. which are functioning ok within the production PayPal environment i.e. non-sandbox
I look forward to any feedback, and please let me know if you need anymore details.