I've got a page containing a form block. When I edit the form block, and press "Update", it works across all browsers (including Explorer 7!), but in Explorer 8 I just get a blank page with the following text:

{"aID":"46","arHandle":"Pagina hoofd content","cID":"111","error":false,"bID":"996"}

Right before this is shown, I can see a Javascript error popping up:

"Webpage error details

User Agent: Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 6.1; WOW64; Trident/4.0; SLCC2; .NET CLR 2.0.50727; .NET CLR 3.5.30729; .NET CLR 3.0.30729; Media Center PC 6.0; .NET4.0C; .NET4.0E; AskTbFXTV5/5.9.1.14019)
Timestamp: Thu, 24 Feb 2011 10:18:11 UTC


Message: Object required
Line: 512
Char: 17
Code: 0
URI:http://neurologie.osage2.nl/concrete/js/jquery.form.js?v=4fda03c0e7...
"

And yes, is says the user agent is Mozilla/4.0, but I can assure ou it's IE8.

This is using C5 version 5.4.0.5.

Any ideas on what's happening?