What is the default client file to look in for errors in a scheduled client...

According to the IBM Spectrum Protect V8.1.9 documentation, the dsmsched.log file is the default client file that contains error logs for scheduled client backups. This file is located in the directory where the Spectrum Protect client is installed.

The dsmsched.log file contains detailed information about scheduled client backups, including any error messages that may have occurred during the backup. This file is a useful tool for troubleshooting issues with client backups.

https://www.ibm.com/support/pages/collecting-data-ibm-spectrum-protect-client-backup-and-restore