Thank you for the tip !
Solution 1 found
I have changed this in my Class defintion
From
System.Xml.Serialization.XmlRootAttribute([Namespace]:="http://www.sigedis.be/db2p/declarations/odse/cbss-sigedis/v1", IsNullable:=False)>
to
System.Xml.Serialization.XmlRootAttribute([Namespace]:="http://www.sigedis.be/db2p/declarations/odse/v1", IsNullable:=False)>
Solution 2 found
I have CHANGED the input file with find replace
Dim fl As String = My.Computer.FileSystem.ReadAllText("responsetest.XML")
fl = fl.Replace("xmlns:db2p-declarations-odse=""http://www.sigedis.be/db2p/declarations/odse/v1""",
"xmlns:db2p-declarations-odse=""http://www.sigedis.be/db2p/declarations/odse/cbss-sigedis/v1"""
)
My.Computer.FileSystem.WriteAllText("responsetest.2.XML", fl, False)