Solution : https://service.sap.com/sap/support/notes/447345 (SAP Service marketplace login required)
Key words : 
attached program correction, terms planned order, planned order conversion, read master data, modification note reason, planned order, master data, modification note 391846, program error, production order
Related Notes : 
       
| 437255 | CO40 with OCM: Automatically functions executed twice | 
| 391846 | Modif: Reading master data when converting planned order |