You are backing up Office 365 mailboxes with cloud agent.
Backup completes with warning:
Cannot back up Office 365 mailbox item: An internal server error occurred. The operation failed. The value is set to empty Parameter name: itemId
It is safe to ignore this warning, the mailbox specified in the error message is backed up.
The issue has been fixed in Acronis Cyber Cloud 8.0, please update to the latest build.