If it's important for you here's a list of all add-ons we iinstalled:

One question : have you tried to replace the false <else> by <else /> in the mgc_cb_evo template ?

Ahm, no, not yet. Where do I have to replace it?

Look for the else word. The one <else> replace by <else />
And also look for the <span> tags, there is one that it is not closed: </span>. Here:

&nbsp;<span>[<a href="$vboptions[bburl]/mgc_cb_evo.php?$session[sessionurl]do=view_archives&page=1">$vbphrase[mgc_cb_evo_archives]</a>]</span>

The </span> is missing in the template.

OK, just replaced the <else> with <else /> but the problem is the samed. The chatbox with IE look like this

Did you try to reduce the % of the chatbox? Maybe you have to do it since it is integrated in a portal page.

Just tried it without success. Still the same problem.

And the </span> tag? Did you correct that? And the changes in this post here.
Did you chnge that too?

All the changes are made without success.

Well, sorry for no help. Maybe VBDev can help you further.