Created Unassigned: Modalpopup IE11 [27663]

I have a problem with the modalpopups in IE11. Currently we are still using IE9 because of this issue. The problem is that the modalpopup windows don't close in IE11. We are using version 3.5.7.0725 of the ajaxcontroltoolkit.dll

Anybody a solution for this problem?

Thanks.