Just opening a page with RadEditor in such device the following errors are thrown:

Error while executing filter FixUIBoldItalic - TypeError: Unable to get property 'getElementsByTagName of undefined or null reference

and

Error while executing filter ConvertFontToSpanFilter - TypeError: Unable to get property 'ownerDocument' of undefined or null reference


For the time being you can disable the filters to workaround the issue:

protected void Page_Load(object sender, EventArgs e)
{
    RadEditor1.DisableFilter(Telerik.Web.UI.EditorFilters.FixUlBoldItalic);
    RadEditor1.DisableFilter(Telerik.Web.UI.EditorFilters.ConvertFontToSpan);
}