So your .htaccess file looks like this?
<IfModule mod_rewrite.c>
RewriteEngine on
RewriteRule ^$ app/webroot/ [L]
RewriteRule (.*) app/webroot/$1 [L]
</IfModule>
You also have the correct info set up in your app/Config/constants.php for the subdirectory? Should be nameofdolder/ with the tailing slash.
The error logs for the .htaccess are going to be in the general error log location for the server. Depending on the flavor of linux /var/log/httpd/error_log .
If you are still having an issue your host should be able to give you the error that is being written to that log. You will not have access to it.
Jason