Try this
create a text file with few bytes in it and give it this name
2682_E_test_0807-01_L_bidr_betterplan.txt
then copy this file in windows and paste
the file name will be
2682_E_test_0807-01_L_bidr_betterplan-copy.txt
then copy those two files and paste
file name will append copy to the file name
this way you will have longer file name
create only 300 to 400 files (each time you paste ,select all files again and paste it will take you less than a minute)
attempt an upload for those files and se if it works
i think the problem
has to do with the length of the file names
there seems to be a predefined buffer size for ajax uploader control
if the sum of bytes of the selected file name is bigger than that buffer size it fails
Let me know