Fred,
FWIW, I've just encountered a similar error.
The solution, I found, was to make sure that your calling control updates the window control in your ajax settings.
In my case, I have a button that is supposed to open a window that uses a ContentTemplate, which means it can't be in a RadWindowManager.
I was just using this ...
to open the window, but was not getting anywhere. The script function, incidentally, looks like this ...
All I needed to do was add a new set of AjaxSettings to my AjaxManager ...
I'm guessing here, but I wonder if replacing "btnBeginLogin" in the above with the name of your AjaxManager control would fix your problem.
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Stuart. If this answers your question, please don;t forget to use the "Mark as answer" link.