"log error debug info warn assert clear dir dirxml trace group" seems to be a list of method available for javascript console. Odoo itself did capture and process console.log so that it can be shown in it's error pop-up.  Maybe your client is using some browser plugins or other software  that changes the way javascript console works?

Perhaps your client is able to make a screenshot? We have several clients using win7 and IE to connect and have not had similair problems yet. Does it work on other browsers?

Ludo,

I've asked screenshots but he says the browser itself shows nothing.

My guess is that their proprietary security software causes the problem. But I needed some confidence before I assure him.

Thank you anyway.