Error: Program/Area: paymentTxnExtShareLockProcessor.paymentLock, Subject: Database.update(paymentTxnUpdList)
When an online payment has been made, a field called "Allow External Sharing" is set to being checked.
This means the Payment Txn can be shared with the public website. This is needed so the Payment Txn can be retrieved by the Checkout page to shown details such as Receipt Number on the complete page.
About 4 times a day, a processor called "paymentTxnExtShareLockProcessor" runs that then removes this checkbox so that the Payment Txn is no longer able to be externally accessed by our forms.
What can happen is a validation rule or restricted picklist can cause the updates to fail. If you locate the Payment Txns shown in the error and do a simple edit/save, then you'll get exactly the same error message as logged.
You will need to correct the data/validation issue or disable the Validation Rule in question. As a side note, if using custom Validation Rules, please do review our System Admin FAQ on Validation Rules.