Hammamet,
I had the exact same situation. I had a user with a corrupted mailbox and I simply removed their mail box from the backup. Give this a shot and I'm sorry if it may sound basic but simplicity may deffinitely help:
1) Open Backup Exec
2) Top left corner click on Backup
3) Under the Properties section on the left, look under Source - Selections.
4) Under Selection List Name, select your backup job from the drop list.
5) Expand Microsoft Exchange Mailboxes
6) Remove the check beside the user you think may have issues with.
7) Select Run Now and see if the job runs.
I tested it, after making the change and verified my overnight back and it worked great. Remember to make sure all the necessary services are started/running.
Good luck,
TZeus