after login cms freezes

[Foton]

Here is a strange thing;
all worked fine on my PC and of my client. But suddently he can not login anymore. After entering his login and password, the page is trying to open, but nothing really happens...On whatever PC I am trying to enter the CMS, everywhere the same problem.

To refresh the login page to enter a different user, I first have to close down the browser (IE6) otherwise the same error pops up. While sitting next to the server of the network, all works fine.

So something is holding up the admin-pages. Has anyone any cleu what is going on and how can I get it to work again. Is it a virusscanner/proxyserver/cache thing?

Any help appreciated.

[Foton]

It is not a CMSMS issue.
When connecting from the PC to the server and from there to the login page, all is working fine.
But still when browsing from the PC to the login -page nothing happens.
Very strange and I really do not have any clue what is going on....