403 forbidden for all css files located in /modules My multi site Drupal 7 installation is suddenly unable to load any CSS file located in the /modules folder and subfolders (for example http://example.com/modules/system/system.base.css). I get "Failed to load resource: the server responded with a status of 403 (Forbidden)" in the browser console.

All CSS files for installed custom modules and all custom theme CSS files (inside /sites/all/modules and /sites/all/themes) are loading just fine.

Even more strange: the /modules/ CSS files load fine if I enable "Aggregate and compress CSS files." in Configuration > Development > Performance. When I disable "Aggregate..." I get the 403.

I realize that aggregating CSS is recommended in production sites, but I need to figure out what is going on and why the CSS files are not loading (they were loading fine earlier today). I did some changes in permissions earlier today, but I reverted those, with no success.

Any ideas?

PS: I noticed that Drupal adds "?obkukf" after .css - what is this, and does this have anything to do with the problem?
That extra query string are used to control caching. That will change everytime the cached files expired. 

As per Dave Reid: 


  It's to prevent browsers from caching those files when they change.


See these threads: 

https://www.drupal.org/node/346711
https://stackoverflow.com/questions/11362261/appending-query-string-to-all-js-and-css-files-to-prevent-caching