Hi, Matt Shipman
Good day!
Thank you for posting to Microsoft Community. We are glad to assist you.
According to your description, I'm sorry to hear that your user is experiencing this issue. Based on the error message you provided, it seems that the user no longer has access to a specific task in Board view.
Since the user is able to view and create new tasks in List view, it's possible that the issue is specific to that particular task. Have you tried deleting and recreating the task to see if that resolves the issue?
If that doesn't work, it's possible that there may be a permissions issue with the task itself. You may want to check the permissions for that specific task and ensure that the user has the necessary permissions to access it.
If neither of those solutions work, it is best for you to get in touch with Microsoft Online Technical Support team directly.
For this team which is unlike our forum team, and they have higher permission that can check on the backend from the server-side. We believe they would able to help troubleshoot the issue and provide more specific guidance.
To get in touch with Microsoft Online Tech Support team, please ask your administrator and follow the below steps:
Ref- Get support - Microsoft 365 admin | Microsoft Learn.
Thanks for your precious time and your understanding would be highly appreciated.
Hope you all the best!
Alex | Microsoft Community Moderator.