As per the subject, I have an Approval Task sent to an external user and he gets the email without problem.
Then clicks the link to the Workfow Task: it opens fine, switch to Edit move and clicks the Radio Button to "Approve", then clicks OK: the following message is displayed in Red
"Access denied. You do not have permission to perform this action or access this resource."
I checked the permission in Workflow Tasks list and even tried to elevate it to Full Control and still error.
Then also checked the special list /Lists/Workflow%20History and still had enough permission too.
thank you for throwing some suggestion that I didn't think of. (must be something related to that external user...)