A workaround which can be used for the moment is to traverse the TextFragment elements in the affected pages and set their Clipping to null:

foreach (ContentElementBase element in page.Content)
{
    TextFragment text = element as TextFragment;
    if (text != null) text.Clipping = null;
}

Error: "We found a problem with this formula. Try clicking Insert Function on the Formulas tab to fix it." appears on the screen when a user tries to:
1. Selects a cell.
2. Inserts Sum formula from the ribbon menu by following Formula --> Auto Sum --> Sum.
3. Selects Range.

If RadSpreadsheet is shown in RadWindow or in Popup, Filtering dialog throws NullReferenceException.