Specified a domain name in front of the samaccountname. For example just http instead of I\http.
Please rerun the/opt/quest/sbin/setup-mod_auth_vas script again and enter just the samaccountName when asked unless this is a cross domain environment.
If it is a cross domain environment you would enter double forward slashes so the first one is escaped like the following: I\\http
In oneway trust environment, you may have to modify the service account to add appropriate permissions. For example go to Active Directory, find the account created by setup-mod_auth_vas, click "security", add "crossforest_domain\Domain Users", and check "Allow to Authenticate".