In vCO 5.5.2, when you get error messages “ch.dunes.vso.sdk.SDKTypeConvertor_server$1 cannot be cast to XYZ” or “Unable to serialize object of class : ch.dunes.vso.sdk.SDKTypeConvertor_server$1”, you hit a bug in the vCO platform.
This issue for example occurs if you use the vCAC Plugin, to run vCO workflows as part of vCAC lifecycle processes. Then the “workflow runner” workflow in the Library fails with the error (see screenshot)
To solve the problem, you have to install a patch, the description and the patch download you can find in the VMware KnowledgeBase:

Steve Jin: VMware VI and vSphere SDK
David Flanagan: JavaScript – The Definitive Guide
Marijn Haverbeke: Eloquent JavaScript
Cody Bunch: Automating vSphere: With VMware vCenter Orchestrator