When uploading files via the Bulk Uploader, logged in as any user besides admin, I'd get the error:
"Error uploading file filename.extension: Invalid user"
After some investigation, it appeared the $_COOKIE wasn't being set properly. It was set before sess_read, but inside sess_read it was NULL, and caused the above error.
Instead of checking if the user is logged in via sess_read, I changed it to check to see if the global user id is greater than 0 and it works great.
Others have had this problem:
If you think this is a good solution, please consider the attached patch for the next release.