Solution : https://service.sap.com/sap/support/notes/202593 (SAP Service marketplace login required)
Key words : 
backup failsthe savr3sys command offers, sap system work processes reach, surprisingly high offline time, i5/os release v5r3m0, schedule long-running transactions, bc - sap database management, long-running batch jobs, os/400 backup commands support, dscr3sys und rcnr3sys commands, backup file qgpl/savr3sys
Related Notes : 
       
| 825473 | IBM i: Online backup as of V5R3M0 | 
| 484548 | iSeries: 3 tier system installation | 
| 214369 | iSeries: 3029, Resource busy, /usr/sap/<SID>/SYS/global |