Solution : https://service.sap.com/sap/support/notes/542918 (SAP Service marketplace login required)
Key words : 
windows environment variable $temp, web application designer crashes, web application designer, individual program steps, program crashes, 'temp' directory, terms trace, crash reason, behaves incorrectly, 'trace' file
Related Notes : 
       
| 542933 | Runtime error '429'.'ActiveX Component can't create object' | 
| 533542 | WAD crash due to a corrupt workspace file |