Something is mixed up badly somewhere with the API setup on this site. It will be best to do a clean up and setup new. Do the following:
#1) Delete the API related settings and the webhooks from settings menu of the plugin.
#2) Delete the products
#3) Delete the app from the PayPal developer portal.
#4) Follow this documentation to setup the app and the API details using your new PayPal account.
#5) Setup a new product and test a transaction:
#6) For the subscription type products, The most important step will be to delete any of the old subscription products that you may have. Trying to reuse the existing subscriptions products won’t work as PayPal requires the subscription plan to be newly created after API details are changed. So, after you have set up the API details, create a new subscription product and then test it.
I will reply to your email also.