After some testing I have found the following errors in your code.
1) WP-Minify uses minify engine. I hope you frequently update it as it can have errors. ie
2) WP-Minify breaks if the following values are set in wp-config.php with a trailing slash. but works fine if not
define('WP_CONTENT_DIR', '/home/username/public_html/wp_content' );
define('WP_CONTENT_URL', 'http://www.example.com/wp_content' );
THIS DOES NOT
define('WP_CONTENT_DIR', '/home/username/public_html/wp_content/' );
define('WP_CONTENT_URL', 'http://www.example.com/wp_content/' );
I would suggest testing if WP_CONTENT_DIR and URL have trailing slashes if using it breaks the plugin
3) Some servers get an error when using wp minify with the following error:
Warning: realpath() [function.realpath]: open_basedir restriction in effect. File(/usr/local/apache/htdocs) is not within the allowed path(s): (/home/:/usr/lib/php:/tmp) in /home/username/public_html/wp_content/plugins/wp-minify/min/lib/Minify/Controller/MinApp.php on line 107
I have fixed this problem by manually setting in min/config.php (line 65)
$min_documentRoot = '/home/username/public_html/';
To be honest I think it really sucks to not be able to specify the caching directory. I would really appreciate it if you could code this in some time.