Tux:
The customer reported that, on the PCs where the error occurs, uploads of files with a size smaller than 150 KBs are always succesful, but a greater size than that generates the problem. We still have to verify this feedback, but it might be a clue.
It's confirmed, the file size makes the difference. In some PCs of the customer (not all of them), if they upload a file smaller than 150 KBs, the upload works fine, if the size is greater then 150 KB..... error!