The cause for such a problem would be one of the following:

1) The Web server does not appear to have the FrontPage server extensions installed. You can install your FrontPage extensions by following the steps described on the following URL:

http://www.siteground.com/tutorials/frontpage/frontpage_extensions.htm

2) The _vti_bin virtual directory is not marked executable. In this case, you need to change its permissions. You can change the permissions from:

cPanel -> File Manager

Once you have found _vti_bin directory, click on its name and then click on Change Permissions in the upper right corner of your screen.