Hi Michal,
The error I get in visual studio and the web:
Visual Studio show this error in debugging: [BasicForm.DataRow]:
This property can be read only if it was previously initialized through set.
Web: System.Exception: [BasicForm.DataRow]: This property can be read only if it was previously initialized through set.
So what I found out in debugging the following line is the problem when it try to update the value:
DataHelper.SetDataRowValue(formElem.BasicForm.DataRow, col, client.Translate(ValidationHelper.GetString(node.GetValue(col), ""), fromCulture, toCulture));