Make sure that a domain name doesn't contain characters in upper case. If it does, you should rename the domain.
For example if domain name is "Domain.com" you can rename in to "Domain1.com" then to "domain.com" and reinstall FP for the domain.
Also make sure that FP Authorization is enabled on domain hosting setup page.
If it still doesn't work, take the following steps:
1. disable FP support for domain;
2. run the following commands on the server:
find . | grep /.htaccess | while read f; do rm -rf "$f";done
find . | grep /_private | while read f; do rm -rf "$f";done
find . | grep /_vti_ | while read f; do rm -rf "$f";done
3. enable FP for domain.
Also, you may track a real problem down by using the following command lines:
/usr/local/psa/admin/sbin/websrvmng -v --install-fp --vhost-name=domain.com --admin-name=fp_user --user-name=ftp_user --port=80
4. also, please look through the domain's httpd log file for error messages and if you see the following message:
no acceptable variant: /var/www/error/HTTP_UNAUTHORIZED.html.var
then, probably, it means that the file HTTP_UNAUTHORIZED.html.var was overwritten with the default one that comes with httpd package. In this case you have to restore the original HTTP_UNAUTHORIZED.html.var file that comes with frontpage RPM.
It could be done with the following command lines:
#cp -a /usr/local/frontpage/version5.0/HTTP_UNAUTHORIZED.html.var /var/www/error/HTTP_UNAUTHORIZED.html.var