Remote account objects not scanning
When you link account you have to deploy a cross-account role in the remote account. You will see a list of the linked accounts and their status on the Link Accounts page. If a remote account was previously setup with the cross-account role and scanning successfully, then there is an issue with the role in the remote account. It may have been deleted by an account admin by accident or numerous other reasons as to why there may be an issue.
Here is what it will look like if Antivirus for Amazon S3 can no longer utilize the cross-account role to get buckets:
The first indicator that tipped you off may have been the Problem Files page results or a Proactive Notification.
In order to fix the problem, you have to fix the cross-account role. This may be able to be done in the remote account directly, but you may just need to start over with the role. If the stack still exists in the remote account you could simply rerun the stack there. Alternatively, if you need to run the stack for the first time or again from scratch, you can click the action button on the remote account row and select
Launch Stack as seen here: 
This will launch the
Quick Stack Create wizard in whichever AWS account you are currently logged into. Ensure you are logged in to the remote account in question.