Hello Daniel Botero Correa,
Is your RadAsyncUpload ajaxified using RadAjaxManager? Such error is caused sometimes when RadAsyncUpload is axafied with RadAjaxManager/RadAjaxPanel.
RadAsyncUpload provides its own mechanism for AJAX interoperability so one don't need to ajaxify it additionally.
Could you please try removing RadAsyncUpload from the ajaxified controls, does the issue persist?
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