Embedded white space in nagios arguments

When you are executing the command manually, you are calling /usr/lib/nagios/plugins/check_http correct?

When you are calling check_http from the nagios config, it looks at /etc/nagios-plugins/config/http.cfg to determine the check_http command arguments.

What you need to do is create a custom check like:

define command {
    command_name                   check_web_content
    command_line                   $USER1$/check_http -H $HOSTADDRESS$ -t 60 -w 15 -c 45 -u $ARG1$ -s $ARG2$

}

Then in your service configuration your check_command should be something like:

check_command check_web_content!http://www.domain.com/index.html!"Text on site"

I don't see it mentioned in your question or any of the answers but have you checked the nagios.cfg file for the enable_notifications setting? Is it set to 0 or 1?

Also is this only happening on a given service notification or is it affecting all notifications? If it's all I would suspect that enable_notifications is set to 0 in the nagios.cfg file. If so try changing it to 1 and restarting Nagios.