Sending e-mail with new comment failed:

SendEmail failed. First exception on row 0; first error:

INVALID_CROSS_REFERENCE_KEY, invalid cross reference id: []

1) This error is indicative of a problem with the Organization-Wide Address. One of the following conditions probably exists: 

• An Org-Wide Address was never set.
• The Org-Wide Address was deleted and a new one was not selected in the Email to Case Premium Outbound Setup.

Check Setup > Email Administration > Organization-Wide Addresses and verify that each address you are using with E2CP exists and is verified. 

Navigate to the E2CP Outbound Configuration and configure an available Organization-Wide Address.

2) The Contact configured for Contact Substitution is not defined in the setting in the Setup Wizard or has been deleted.

• In the Setup Wizard, make sure that there is a Contact defined under Global Setup - Contact Substitution, and it is a valid Contact and has not been deleted.

 

This issue is due to misconfiguration.

See Salesforce Help & Training for more information about Organization-Wide Addresses.