Please check the first 3 digits of the id number returned in the error message: First exception on row 0 with id 000xxxxxxxxxxxx

This 3 digit prefix identifies the Object for which the error is occurring.

Cross-reference the 3 digit entity ID with this list of Standard Object Entities with their IDs to determine the object for which the deletion is failing: http://help.salesforce.com/apex/HTViewSolution?id=000005995&language=en_US

If its not in there, it is probably a custom object and here's how to determine which:

Next, ensure that you have given the affected user sufficient permissions to the object that you wish to delete upon merging cases; permissions can be assigned via profile or permission set.

For example, if Delete Original for Attachments is enabled, which is common, the user performing the merge will need either own all of the cases they are merging, have delete permissions on Case, or Modify All Data org wide. https://help.salesforce.com/HTViewHelpDoc?id=notes_del.htm

As a second example, if Delete Original for Tasks is enabled, the user performing the merge will need the permissions described here: https://help.salesforce.com/HTViewHelpDoc?id=activity_del.htm 

(It is also important to check your Case Merge Premium Settings to verify that you actually want to delete this specified child object from the duplicate case(s) upon merging.)