Joomla cache causes backend login problems

[Funtshirts]

Turning on joomla cache causes this error on the backend login page/form.
And i cant find anything like addevent in my index.php file like it says.
Can anyone explain to me how i can fix this and how its possible that joomla cache causes this??
my website is www.funtshirts.nl

TypeError: window.addEvent is not a function
   

window.addEvent('domready', function ()

   
index.php (line 44, col 3)

[jenkinhill]

Have you asked on http://forum.joomla.org/ ?

[Funtshirts]

Will do now.