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.
PS: I noticed that Drupal adds "?obkukf" after .css - what is this, and does this have anything to do with the problem?