Junk mail - count is stuck, messages invisible

Hello -

Suddenly, my junk mail box is stuck. It shows that there is '1' unread message, but when I click on it, the mailbox appears to be empty. When new junk mail arrives, the count number goes up, but the mailbox still appears to be empty.

When I click the gear wheel at the bottom and select "Erase Junk Mail", the count goes back down to '1' again and stays there.

Quit and Restart don't solve the problem.

Also, when the problem mailbox is selected, the 'Rebuild' option is grayed out (rebuild is not possible). For all other mailboxes, the 'Rebuild" option is allowed, (not gray).

Any suggestions?