Weird Problems with Moodle 2.5.1

I have a client using Moodle. We have just upgraded to the latest version 2.5.1 and a puzzling problem has appeared.

The upgrade went fine and on the console everything works as it should.

Now this tells me that everything is really OK and this is not a Moodle problem. When we access the Moodle from any other machine we are unable to upload a file because we don't get the upload screen. ( as in the image) It just sits there with a timer sitting there on the screen. On slow servers (when the machine is loaded) we see this for a few seconds before the screens load, but in this case that's as far as it gets. Its not the browser as we have looked at several and we have tried accessing the server in different ways on different networks.

The problem is more interesting that a previous version of Moodle 2.4.3 also runs on the same virtual machine (Windows 2008R2) and network and surprisingly that works fine. No errors. But three different implementations of Moodle 2.5.1 on three different virtual servers all fail in the same way. When they had the previous version of Moodle (2.4.3) they too worked OK.

So it appears that the network is ok and the firewall antivirus etc, otherwise the Moodle 2.4.3 using the same mechanism would fail, but the latest version of Moodle on three different machines have the same symptoms and that points to a network issue. So far I have have no response on the Moodle forums and I have had no luck trying to reproduce this problem on my home server network using largely the same software.
No else seems to have the same problem.

I guess that the daily build on Moodle might help to upgrade again.

So it must be Moodle, but its a puzzle that yet has to be solved.