Installing Frontpage from Helm to a site results in only the server wide FP_Admin being able to access the account. The user name chosen by Helm is not actually assigned to the site, so a user cannot use it login to Frontpage on their site.
Helm adds the Frontpage user domain.com_fp to the AdvAuthor group for that domain.
Make sure that the AdvAuthor group has the relevant permissions in Frontpage. Make sure the user web does not contain things like Sub Webs that the AdvAuthor does not have permission to publish (unless altered).
Also ensure the details in the Web service in Helm are correct. The FrontPage admin username and password MUST be an administrator account on the server. Also check the port number is correct.