My best guess is that the document is being locked either by another workflow or by a user. I run into a lot of Entry Locked errors in Workflow because users will try editing something after they've started a business process, and it's hard to catch because by the time you see the error the lock is already released.

To find the culprit, you could try including the Checked Out By and Locked By properties when you get the document, then using a Track Tokens activity to log them; that way if/when it happens you can see if it is workflow or a user causing the issue.

I don't believe Entry Locked is a critical error by default so Workflow should already retry automatically, but you can check your settings in the Workflow Admin Console under Server Configuration > Task Error Handlers and Error Code 9014.