Dataset.GetXml throws an InvalidOperationException when column is type of object. It works in windows, but doesn't work in mono
DataSet s = new DataSet();
DataTable t = s.Tables.Add();
This no longer repros on Mono master where System.Data was replaced with MS referencesource code.
Mono 5.10 has significantly improved System.Data implementation which should resolve this issue. If you can still reproduce it please reopen the issue.