Please restore the web.config file from your backup and verify if the issue is resolved. Additionally, you can find the Release 15 file at the following link. Before applying the patches, ensure you take a backup of the existing code:
https://drive.google.com/file/d/1MipB__SZglk0RqV3elVtHcwL-6KDC_ZI/view?usp=drive_link