Maybe in your php.ini has been setted by default a folder where to upload files, and now that folder doesn't exists.
You can check in your php.ini file, looking this line :
;;;;;;;;;;;;;;;;
; File Uploads ;
;;;;;;;;;;;;;;;;
; Whether to allow HTTP file uploads.
file_uploads = On
; Temporary directory for HTTP uploaded files (will use system default if not
; specified).
;upload_tmp_dir =