If you use a global theme for the RadWindow (i.e. without the skinId) the ReloadOnShow property will transfer to the RadWindow RadSpell uses internally and will modify the querystring that loads the page inside. This should generally be avoided and although it should not be a direct reason for the problem I advise that you set a SkinId for your theme to avoid this undesired inheritance.
Another option is disabling theming for the RadSpell so its RadWindow does not inherit your theme:
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