CSS Page Not Founds

drupal_get_css() automatically checks if the file exists, so you shouldn't be seeing this error. I even tried changing to a different theme that would have caused this same condition and I never got the non-existent CSS included in the source or any reports for a missing vertical tabs CSS file.

have similar log records:

page not found: .../sites/all/modules/vertical_tabs/acquia_prosper/vertical_tabs.acquia_prosper.css

actually such a file presence only in garland theme.
copy+rename from

.../sites/all/modules/vertical_tabs/garland/vertical_tabs.garland.css

changes nothing

solved with upgrade to vertical_tabs-6.x-1.x-dev (2011-jan-27)

whats causing this?

i'm having the same issue.. upgrade to dev didn't work.

/sites/all/modules/contrib/vertical_tabs/NAME-OF-THEME/vertical_tabs.NAME-OF-THEME.css
Failed to load resource: the server responded with a status of 404 (Not Found)

I'm getting this same error message with a Fusion sub-theme. I discovered that by disabling the admin_menu module (6.x-1.6), the log messages stopped appearing. So, it appears to be a conflict with the Administration menu module.

Well, OK, it's apparently more peculiar than just a conflict with the admin_menu module. I noticed that the problem only occurred for me when adding a node for a content type that has both a tagging module widget and a multi-select vocabulary while the admin_menu is installed. Disabling any one of those three configurations caused the errors to stop appearing. A bit of an edge case but it appears to be consistently reproducible.

It's very strange... Don't think it's related to Vertical Tabs in any way.. Rather another module screwing up something.

Meanwhile though, I just put a temp fix in Vertical Tabs for it:

change line 362 in vertical_tabs.module to:

      if (!$theme_stylesheets || !module_exists('color')) {
        // The theme-specific CSS will be only included by drupal_get_css() if
        // it exists so we do not need to check file_exists() here.
        // EDIT: appearently something wrong, so we do check here:
        $path = drupal_get_path('module', 'vertical_tabs') . '/' . $theme . '/vertical_tabs.' . $theme . '.css';
        if (file_exists($path)) {
            $css[] = $path;
        }
      }

Ok I committed a fix with http://drupalcode.org/project/vertical_tabs.git/commit/d426c3e that should solve this.