Problem with only registered

Hi,

I’ve found the puglin “Only registered” to limit the access to my private blog. It seems to be exactly what i’m looking for, but when i activate it, i have a problem on my administration page. The following inscription appears : “Warning: Cannot modify header information – headers already sent by (output started at /home/httpd/vhosts/domainename/httpsdocs/blog/wp-content/plugins/registered-only.php:2) in /home/httpd/vhosts/domainename/httpsdocs/blog/wp-admin/admin.php on line 10

I don’t know why.
I use the WP version 1.5.2.
The blog is installed in an https directory.
Thanks for your advices and your help.