I changed my PayPal Manager (and hence Website Payments Pro) password today and then started receiving this error when viewing any transaction paid via PayPal.

I wasn't sure what was causing the error and couldn't find a thread explaining this specific problem so am posting this in case anyone makes the same silly mistake.

PayPal Manager has 2 passwords - a PayPal Manager Password and a Website Payments Pro password. I think by default they are the same, but you can make them different if you choose to do so.

PayPal Manager>Account Administration>Manage Security>Change Password

If you change the default password it will change both PayPal Manager and Website Payments Pro passwords and then you will need to change the WPP password in Zen-Cart>Modules>Payment>PayPal Express Checkout.