Solution : https://service.sap.com/sap/support/notes/445507 (SAP Service marketplace login required)
Key words : 
additional key words rftbup01, business partner conversion phase, lock table overflows, source code change, business partner, conversion report, deletion flag, unlocked anymore, large number, system terminates
Related Notes : 
       
| 541293 | SAP BP conversion: Data inconsistencies after BP conversion | 
| 398888 | BP_TR1: Composite note and FAQs about bus partner conversion |