Solution : https://service.sap.com/sap/support/notes/115356 (SAP Service marketplace login required)
Key words : 
error message 'wbs element <external, additional key words, wbs internal/external, wbs element, external wbs, internal wbs, release 31h, joint venture, bank switching, direct input
Related Notes : 
       
| 129320 | IS-OIL: Applying LCPs 29 to 32 on R/3 3.1H | 
| 129319 | IS-OIL: Applying SPs 29 to 32 on R/3 3.1H | 
| 115508 | update error if switch doc which is not in t8jvba | 
| 114657 | VBA different enhancements/problems for rel. 31H |