Hello Roudolf,

We have released a service pack that addresses the issues. The latest update is available and you can install it via the Control Panel or download it manually from the AJAX Downloads page.

I hope you enjoy the updates! Feel free to reach out and keep me posted.

Regards,
Vasko
Progress Telerik

Hello Aurel,

Thank you for reporting the issue!

We have looked into this problem and we found that some of the new changes did not take the RenderMode into consideration.

The problems seems to appear when the RenderMode of the Window set to Classic and loading the content through the NavigateUrl. We found a fix for this problem and will be released in a hot fix at the beginning of next week. 

Workaround

Until the issue is resolved, please use the following workaround:

Add the following JavaScript code anywhere after the ScriptManager declaration:

<script>
    (function ($) {
        try {
            var $T = Telerik.Web.UI;
            var original_window_createUI = $T.RadWindow.prototype._createUI;

            $T.RadWindow.prototype._createUI = function () {
                original_window_createUI.call(this);

                this._contentElement = this._contentElement || this._contentCell;
            }
        } catch (e) {

        }
    })($telerik.$);
</script>

We apologize for any inconvenience this may have caused.

Regards,
Vasko
Progress Telerik