Did you get an Access Denied message while publishing a solution? The problem is likely the Async service account. If the service account does not have full access to the %Program Files%\Microsoft Dynamics CRM\CustomizationImport folder, you will get the following errors:
1. Open properties of Customization Import Folder on the backend server with Async service assigned.
2. Edit the permissions by clicking "Edit", then click "Add". Find the Async service user, click OK, then click on the Full Control checkbox "Allow".
3. Re-run the import operation.