Depending on your system configuration and your Role Hierarchy, it is possible to receive this error due to an Editable Source selection on the Campaign. In a multi location Organization with Sharing Rules, there can be many marketing Sources.
The Call Queue code queries all Sources with the same Source Type as the Lead Source record you are about to call. The tool does not apply any Sharing Rules and therefore Campaigns cannot have "Source Is Editable" checked. Keeping the box unchecked assures the system is not trying to query Sources that are not shared with the Call Center User.
Resolution:
Edit the Campaign causing the Error. Uncheck the 'Source is Editable' checkbox (set to False). Save the Campaign.
