Solution : https://service.sap.com/sap/support/notes/768971 (SAP Service marketplace login required)
Key words : 
user context reason, start transaction se11, support package higher, terms ace group, ace group calculation, ace group entries, ace group tables, 'unique index' setting, ace authorization, ace groups
Related Notes : 
       
| 770499 | Index conversion of ACE group tables not possible | 
| 761607 | CRM 4.0 SP Stack 9/2004 (SAPKU40007): Release & Info. Note |