If your Call Center is located in Canada, be sure the User record has a Locale setting of "English (Canada)"
A Canadian Org setup will have dates formatted as DD/MM/YYYY. When a User in the Call Queue attempts to Complete a Call Disposition, the system is entering the date as formatted for that specific User. If that User's Locale setting is "English (United States)", the date formatting for the User is: MM/DD/YYYY. Because the date formats are different, the system will not accept the date and the Call Disposition will automatically fail.
Changing the User's Locale Setting to match the Organization-Wide Locale Setting will resolve this issue.
The Organization-Wide Locale Setting can be found by navigating to:
Setup > Company Information > Edit
Note the Default Locale of the Organization
Next, check the User's Locale Setting by navigating to:
Setup > Users > choose User > Edit
The Locale Setting of the User needs to match the Organization's Default Locale Setting (regardless of Time Zone):