Infragistics sent me a workaround:

1. Install the latest hot fix (2007 V3 or 2008 v1).

2. In the BeforeSortChange event, add the following: e.ProcessMode = ProcessMode.Synchronous;

This fixed the test app that I sent them, but our production App still would hang up calculating summaries when re-sorting. I found an additional workaround which, in combination with the above, eliminated this behavior:

 3. In the BeforeSortChange event, remove the summarycolumns that are of type SummaryType.Formula, using code like the following:

Light at the end of the tunnel ....?

 I think this bug is not connected with custom formulas.

For me the necessary conditions for reproducing are:
There are many rows in grid (> 1000) and 3 or more columns are "grouped by".
If I clear the data source and fill it with the new data, the application hangs.

Here is a workaround I use:

I've created the following class:

public class SwitchOffGroupByCookie : IDisposable
{
    private readonly UltraGrid grid_;
    private readonly UltraGridLayout layout_ = new UltraGridLayout();

    public SwitchOffGroupByCookie(UltraGrid grid)
    {
        grid_ = grid;
        layout_.CopyFrom(grid_.DisplayLayout);
        grid_.DisplayLayout.Bands[0].SortedColumns.Clear();
    }

    public void Dispose()
    {
        if (layout_.Bands[0].SortedColumns.Count > 0)
        {
            foreach (UltraGridColumn col in layout_.Bands[0].SortedColumns)
            {
                grid_.DisplayLayout.Bands[0].SortedColumns.Add(col.Key,
                                                               (col.SortIndicator == SortIndicator.Descending),
                                                               col.IsGroupByColumn);
            }
        }
    }
}

using (new SwitchOffGroupByCookie(grid_)) 
{
    dataSource_.Rows.Clear();
    // fill data source with the new data
}