Tabbing does not work with ListSearchExtender (any more)

I am using the ListSearchExtender on a bunch of asp dropdownlists, but now something has happend or a bug has been found  which ever came fist.

The problem seems to only exist in IE as the controls/pages works fine Chrome, FF and Safari.

Action to problem:

When tabbing to the list and typing on the keyboard and tabbing out : all is finebut

1.  when tabbing to the list
2. using up/down to find the correct entry
3. tabbing out
4. -> webpage hags until IE reports some "asynchronous communication timeout"

(The website has recently been updated with Ajax toolkit  ver. 4)

<asp:DropDownList runat="server" TabIndex="3" ID="adresseDropDownList" AutoPostBack="true" OnSelectedIndexChanged="adresseDropDownList_SelectedIndexChanged"></asp:DropDownList><act:ListSearchExtender ID="adresseDropdownListSearchExtender" runat="server" PromptText="Skriv for at søge..." QueryPattern="Contains" IsSorted="true" TargetControlID="adresseDropDownList"></act:ListSearchExtender>