Paypal API error

Since last week or so, users who tried to use paypal to pay, could not longer do it successfully. An message ‘An error occurred while calling the Paypal API’ appearing at the top of the checkout page.
The order appeared as ‘Pending Payment’ in WooCommerce.
We contact Paypal, the payment did not go through and after a day or so, paypal refunded the payment to user. We tried to make sure the API username and password was correct by copying the info from our paypal account to the plugin settings. We have enabled logging as well. I had turned on the log messages. But since it contained password and private information, I did not post them here directly. If you need them, I can send privately.

I do not see any obvious error messages from the log, or anything obvious in the web server log. But I might miss something. Could you help to see what was wrong there? We really need to get the paypal payment working again.

Thanks
Peter

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