Solution : https://service.sap.com/sap/support/notes/572139 (SAP Service marketplace login required)
Key words : 
systemwide work process overview, status  reason stopped sleep, client copy hangs, enqueue reason, status = 'a', target client, transaction sm66, entries contained, data browser, transaction se16
Related Notes : 
       
| 491923 | CC RELEASE: Client copy errors in Release 6.20 | 
| 432152 | CC-RELEASE: Error in client copy in Release 6.10 | 
| 206523 | CC-RELEASE: Client copy errors in 4.6C and 4.6D | 
| 161495 | CC-RELEASE: Error in client copy in 4.6A and 4.6B | 
| 22514 | CC-INFO: Error analysis for client copy |