I'm having the same issue, but with a hierarchical dataset in a many-to-many relationship.
TASK|--< TASKSTEP >--|STEP

My data adapter is correct as currently, Im removing the Expression from the calculated fields using the code below and can save/update the database.

<CODE>
//Clear the calculated columns
foreach(System.Data.DataTable dt in hierarchicalDataSet.Tables)
    foreach (System.Data.DataColumn dc in dt.Columns)
    {
        if(dc.Expression.Length > 0)
        {
            if(dc.ExtendedProperties.ContainsKey("CalcExpression"))
                dc.ExtendedProperties["CalcExpression"] = dc.Expression;
            else
                dc.ExtendedProperties.Add("CalcExpression",dc.Expression);
            dc.Expression = "";
        }
    }
</CODE>

The problem is after the update.  I attempt to place the Expressions back and get an error of
"Object not set to an instance of an object." on execution of :
dc.Expression = strTestVar; in the code below.

<CODE>
foreach(System.Data.DataTable dt in hierarchicalDataSet.Tables)
    foreach (System.Data.DataColumn dc in dt.Columns)
    {
        if(dc.ExtendedProperties.ContainsKey("CalcExpression"))
        {
            try
            {
                strTestVar = dc.ExtendedProperties["CalcExpression"].ToString();
                dc.Expression = strTestVar;
                dc.ExtendedProperties.Remove("CalcExpression");
            }
            catch(Exception ex)
            {
                string str1 = ex.Message;
            }
        }
    }
</CODE>

At first I thought the Expression was invalid, but if I add the following it will add the duplicate column with the Expression.

dt.Columns.Add(dc.ColumnName + "Duplicate" , dc.DataType, strTestVar);

Any thoughts??