Solution : https://service.sap.com/sap/support/notes/630469 (SAP Service marketplace login required)
Key words : 
mmp model mix planning reason, error message /sapapo/seq 840, related note 629393 hass, terms /sapapo/ppc1, /sapapo/seq 840, error message, /sapapo/seq840, program error, enter backflushes, confirmed quantity
Related Notes : 
       
| 629393 | |
| 625253 | Incorrect requirements, lock error, short dump, performance |