Kenneth:
1) Is your site on a load balancing environment?
3) Can you send me the url of your test page abouve? So we can check it too.
1) Yes, it is. We have two Web Servers under load balancement.
2) We will ask them to test it, I'll let you know.
4) Ok, I will read it, and eventually report back to you. But I think that our configuration is correct, since the customer is able (on the PCs where the problem doesn't occur) to upload files even larger than 10 MBs.