Error: "Activity changes could not be saved. One or more of the Contacts or Associates may have been deleted, or this activity may have been deleted" when scheduling an Activity on behalf of another user on a remote database

Symptom: Error: "Activity changes could not be saved. One or more of the Contacts or Associates may have been deleted, or this activity may have been deleted" when scheduling an Activity on behalf of another user on a remote database 

Product Details: 
Product Family: Act!
Product: Act! Pro, Act! Premium
Version: v19

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Cause: This can be caused due to the database schema not completing correctly during the upgrade.

Solution: 

1. Manually run the database schema update script to force the update to complete. For information on manually running the database schema update script, refer to the following knowledgebase article:
   
   How to manually run the database schema update script in Act!
   
   
   
2. Attempt to schedule an Activity on behalf of another user to test