This name is not in a directory

I am testing a Lotus Designer application on Domino server 8.5 FP1 that is running fine on Domino server 6.5. One feature of this application is to pass a minutes of meeting by email to the next recipient. An error window titled “No match” always pop-up whenever the minute is just about to send by email, saying that “This name is not a directory”. The string of the name that is treated as not in a directory for example is “User1/HKPostHKPost”. This actual name that really exist in our domino diretory is “User1/HKPost”. How can this happen? The problem does not exist when the application is running on version 6.5.

Could this be a configuration problem in Domino server? Please suggest a solution.