After a lot of of debugging to see what the VI Java API does internally, I found out that if the web service client (wsc) is invoked with the name of the sublcass instead of the name of the superclass (as last parameter), the response will be converted correctly. This way the slot information can be retrieved without any problems. Here is the solution for those experiencing the same problems:
ClusterDasFailoverLevelAdvancedRuntimeInfo clusterDasFailoverLevelAdvancedRuntimeInfo = null;
try {
final Argument[] paras = new Argument[1];
paras[0] = new Argument("_this", "ManagedObjectReference", clusterComputeResource.getMOR());
clusterDasFailoverLevelAdvancedRuntimeInfo = (ClusterDasFailoverLevelAdvancedRuntimeInfo) serviceInstance.getServerConnection().getVimService().getWsc().invoke("RetrieveDasAdvancedRuntimeInfo", paras, "ClusterDasFailoverLevelAdvancedRuntimeInfo");
} catch (final Exception e) {
//error handling
}
(Note that this only works if admission control failover level policy is enabled!!!)