(60) SSL certificate problem: unable to get local issuer certificate

[one tall man]

I have switched the paypal module to sandbox mode and tried to check out. I did not see anything in the payment methods (no radio buttons or icons at all) but still proceeded to checkout and it was listing paypal. When pressing 'check out' a new window popped up with the red rectangle above the invoice items stating:

We are sorry for the inconvenience. The PayPal account authentication settings are not yet set up, or the API security information is incorrect. We are unable to complete your transaction. Please notify the store owner so they can correct this problem. (10002) 10002 Security error - Security header is not valid

I guess this was due to the sandbox mode and is a red herring, as when I switched over to live mode, I could proceed further to Paypal login page, which is not letting me into the same account as the store owner, which is the only one I have. So I cannot myself test any further. Only one observation: when I am clicking on the big blue Paypal login button, Firefox throws a notification about "clickjacking".