PayPal Capture Error - No Entry in Gateway Log - Can't Process Payments!

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The error I get is as follows:

 

Capture Error

The capture was not successful. Please check the gateway log for more details.

 

The cron runs, but no transactions occur, and when I attempt to manually capture, I get the error above.

 

I'm using the WHMCS PayPal Website Payments Pro payment gateway. Everything was working fine before I upgraded to V 6.0.0 - and I'm still experiencing this issue with V 6.0.1.

 

I enabled module debugging, and manually set debugging in the configuration file, and nothing is reported back for either.

 

I also deactivated, and reactivated & resubmitted the API credentials for the payment gateway - and still, the same issue persists.

 

I'm not sure where to go from here.

[entanglemedia]

As a followup - I decided to see what would happen if I changed the payment gateway API credentials to something I knew was incorrect, and then attempting capture. I expect to see an error in the gateway log - but nothing. It's as if WHMCS isn't connecting to the payment gateway. I can assure you that it is enabled - I double checked in the database.

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As another followup, I reuploaded the files manually to eliminate file corruption, and the issue persists. Can anyone shed some light on this?