Solution : https://service.sap.com/sap/support/notes/389377 (SAP Service marketplace login required)
Key words : 
additional key words s066, credit account -> credit account, transaction se38 > 'create' attributes, symptom update problems occur, ran correction report rvkred77, note system 364928 31i 385091 40a, discovered incorrect credit values, attached correction instructions, incorrect credit values, run report rvkred77
Related Notes : 
       
| 735791 | Composite SAP Note: Credit update incorrect, 500 | 
| 716141 | Analysis: Incorrect open credit values | 
| 666587 | Check for incorrect open credit values | 
| 425523 | Collection of consulting notes: Credit update | 
| 410685 | Composite SAP Note: Incorrect credit update, 4.6B/C | 
| 396338 | Problems in credit management - analysis help | 
| 385481 | Composite SAP Note: Incorrect credit update, 4.5B | 
| 385091 | Composite SAP Note: Incorrect credit update, Release 4.0B | 
| 381789 | Analysis of the update of the credit values using MC30 | 
| 364928 | Composite SAP Note: Incorrect credit update, Release 3.1I | 
| 124571 | Incorrect credit values S066 S067 FD32 |