EE v2.5.5 - Build Date: 20121220 Assets 2.0.5 Matrix 2.5.3 Safecracker 2.1 (not sure if relevant)
I am using (but was not the original developer for) a multisite EE installation. In each installation I have the same channel, field group (Matrix) and identically named image upload path. However, on one of the sites I cannot get images to actually upload to the image upload directory or appear in the file upload dialog, pictured below.