wp-comments-post.php wrong url

I’m experiencing the same issue of this thread http://wordpress.org/support/topic/comments-go-to-a-broken-link-page-1?replies=3

My wp relies in a subfolder but when someone posts it “redirects” to the wp-commets-post.php in the root.
I had comments working until yesterday as the path is correctly set on the settings field.
I tried deactivating all plugins and i’m experiencing the same problem.

EDIT:
solved looking in the comments.php file of my theme and changing wp_url with siteurl. NOW IT WORKS!
QUESTION:
how this could have suddenly happened without touching the theme?