Thank you for the helpful replies. I'm still having this issue. While your answers give good advice I must decipher them since I am not a very techy person and I have little understanding of all these things, especially when it comes to server settings. One thing I noticed is that when I remove the php handler for .html documents in my .htaccess file I no longer see this error and adding a trailing slash to .html pages results in a 404 page. However, in this case my .php scripts stop working (scripts integrated into .html pages, retrieving data from a mysql DB).
This is the php handler I am using:
php_flag allow_url_fopen on
AddType application/x-httpd-php .html .php .htm .shtml