Guido “Trekkie not Star Wars fan” Preite is not the man to puzzle easily but even he was confused:
Someone faced this error before? “Assignee has insufficient privileges” when merging two contacts. The User is System Administrator and the same user is the owner of both records. No plugins running on the merge message. CRM Online DB 8.0.1.344
tl;dr
If you receive “Assignee has insufficient privileges” error message when merging records make sure that no owner of the child records has administrative access mode set.
I want to know why
I’m not sure if it’s possible to display an error message in error but judging by “selected {0}”, this is exactly the case here.
One of the actions performed by the record merge is ever confusing reparenting. For example, in case of the account merge, system will move subordinate contacts from the account being merged to the master account. During that move two things need to happen: master account owner will need to get access to those child records (if they didn’t already have and reparenting rules state that they should) and access of the old child record owner needs to be preserved. It’s the second part that seems to generate the erroneous error message.
The system goes bananas if owner of a child record cannot be explicitly granted access to that record at all, as is the case when user’s access mode is set to Administrative. Interestingly, disabled user accounts do not cause the problem, it’s the administrative access mode that seems to throw the system off.