Salesforce Agentforce Agent Error 'We couldn't retrieve the action output' With Answer Questions with Knowledge Action

The error is thrown because of the Einstein Service Agent User's access to Knowledge.

To resolve the issue, follow the steps below:

1. Provide "Allow View Knowledge" App permission in the existing custom permissions set, which is assigned to the Einstein Service Agent user (OR)
2. Assign the "Agentforce Service Agent Object Access" permission set to Einstein Service Agent User which enables the same permission: "Allow View Knowledge"

If the issue persists after following the above mentioned steps, please contact Salesforce Customer Support for assistance.