Your Kontakt 1.5.3 version had a .dll file in the VST plug-ins folder named "Kontakt.dll". The new Kontakt 2 version, (more than likely) has probably installed itself with name "Kontakt2.dll" in the plug-ins folder.
Your songs that were saved in Nuendo with version 1.5.3 were obviously saved with the "Kontakt.dll" plug-in name (which is now not there) and are looking for that when you launch them now.
If the Kontakt 2 install DID name the .dll file "Kontakt2.dll" (I'm assuming), then perhaps you could re-name the version 2 dll file to just Kontakt.dll. Then Nuendo would find it and hopefully launch the song into the new version 2 that you have.
A safer bet, though would be to either have left 1.5.3 installed and/or "re-load" your samples into Kontakt 2 after launching these songs and then re-save, thus not tampering with the name of the .dll file.