[Plugin: Sidebar Login] Error on activating 2.3.4

I got this:

Warning: include(/var/www/raymondhaaken.nl/_/htdocs/blog/wp-content/plugins/sidebar-login/admin.php) [function.include]: failed to open stream: No such file or directory in /var/www/raymondhaaken.nl/_/htdocs/blog/wp-content/plugins/sidebar-login-2.3.4/sidebar-login.php on line 13

Warning: include() [function.include]: Failed opening ‘/var/www/raymondhaaken.nl/_/htdocs/blog/wp-content/plugins/sidebar-login/admin.php’ for inclusion (include_path=’.:/usr/share/php5:/usr/share/php’) in /var/www/raymondhaaken.nl/_/htdocs/blog/wp-content/plugins/sidebar-login-2.3.4/sidebar-login.php on line 13

Using WP 3.3.1 plus Mystique theme (which has a built in sidebar login, if that matters at all).

http://wordpress.org/extend/plugins/sidebar-login/