Solution : https://service.sap.com/sap/support/notes/763222 (SAP Service marketplace login required)
Key words : 
unicode               42 log      savepoint requested, unicode failed            db_migratecatalog            err            -24895, 53182 migrat   migrating ascii catalog, upgrade sap db version 7, sap db version, unicode            migrating catalog, t4 reason 'shutdown', db_migratecatalog reason, unicode step, migrate catalog
Related Notes : 
       
| 793550 | Supplementary info about the MaxDB version upgrade to 7.6 | 
| 669711 | Additional information about MaxDB upgrade to 7.5.00 |