This looks like a race condition from uploading a file while the server is
on heavy load.
From what I understand, there were 3 different
requests sent to the server while uploading the file.

a) Send content to server
b) Send status information -> determine upload progress
c) Send onUpload event when upload has finished

An ItemFactory will be initiated upon a). In
AuUploader attributes "Attributes.UPLOAD_PERCENT" and
"Attributes.UPLOAD_SIZE" are set at line 515 - 518. If the requests for status
information are processed before these attributes are set (which
happens from time to time on our server) an exception is triggered at
line 115 because "size" is null. What is more, there is another potential
problem with exception handling in line 177 as there is a check on whether
"percent != null". If the CPU switches from thread (request) in step
a) in line 516 to thread (request) in step b) only "percent" is !=
null and "size == null". That means an exception occurred outside the
try-catch-block.

This usually results in a failed upload.

Regards salbader