Export to png does not allow to replacing existing file.

OSX 10.5.6

The first time I do export as png it works great and creates a file with the same name as my balsamiq file. Great.

The second time I try to export as png, it asks me if I want to replace the existing file or save with a new name.

If I click on replace, nothing happens. The dialog just stays open. It does not write the file.

If I manually delete the existing png, it then works.