The Assigned to field was inadvertently changed to String  type, in order to revert it to Reference type (which is the OOB value).

Changing a field type 'string' to 'reference' is possible under specific circumstances.

• All the task records should have a value on the field. 

For example, if there are two records where 'Test String' field has a value saved to it, field type cannot be updated (unless those values are valid Sys IDs).

The record will not display when querying for it through the UI's task.list, it indicates a security constraint restriction error (even with maint role).

Another error message appeared when attempting to modify the Type:

1. Navigate to sys_dictionary.list
2. Search for Table = task & Column name = assigned_to, go into that record.
3. In the Type field, change the current value of "String" to "Reference"
4. Right-click the header and Save

Current behavior: The following error displays in the header:

"Type change not allowed. Invalid type conversion for field 'assigned_to' on table 'task'. Cannot convert from 'String' to 'Reference'. There exists data records (1) that are not a Sys ID."

Expected behavior: The Dictionary form would save without any errors.