Hello. I would like for you to 'REMOVE' (NOT DELETE) the affected sandbox project, then restore it back into Site Admin, as I have seen this resolve issues such as this, when the verify, repair, and project copy doesn't resolve the issue. Note: Removing the project will require a short window of downtime, so if there are any users in the project, they will need to be made aware. I also recommend creating a copy of the project for backup in case any unforeseen issue occurs.
To Remove the project, you will need to right-click on the affected project, and select 'Remove Project'. You will then click 'OK' in the 'Confirm', 'Warning', and 'Information' windows. The project should no longer be displayed in the projects list. In order to restore the project back into Site Admin, you will need to right-click on the Domain name in the projects list, click 'Restore Project' from the list that displays, then in the 'Restore Project' window that displays. You will need to locate and select the dbid.xml file for the project you wish to restore. Once you have selected the dbid.xml file, click the 'Restore' button, click 'Yes' in the 'Warning' window, click 'OK' in the next 'Warning' window, click 'OK' in the 'Information' window, then click the 'Close' button to close the 'Restore Project' window. After the project is displayed back in the projects list in Site Admin, you will need to select the project, then click the 'Activate Project' (Green check mark) button in the toolbar. After you have activated the project, login to the project and see if everything looks and functions as expected. I hope this resolves your issue.