Since migrating from MS CRM 4.0 to 2011 and then immediately from 2011 to 2013, the "Assign" feature is no longer working for some records. We cannot re-assign those records using the Assign button or the lookup field regardless of our security role (even System Administrator). When we try it we get a SQL error and the log file says there is a duplicate activity owner.
We have been able to work around this temporarily by changing the "Assigned To" user from the SQL database directly for some specific records but this is bad practice.
We are trying to figure out how to fix those records permanently the right way. Does anyone have a clue to why it is doing this and how we can fix it permanently?