The reason why Objects.Server works, is because it is in standard namespace (namespaceIndex is 0). The constructor for QualifiedName(String) makes the QualifiedName with 0 as namespaceIndex. The standard namespace is only for UA specification Nodes (e.g. the Server object and default types). The MyDemoObject is not in that namespace, therefore not found.

For nodes not in standard namespace, QualifiedName(namespaceIndex, name) should be used. NamespaceIndex can be obtained from Server using NamespaceTable if you know the NamespaceUri for it, by using addressSpace.getNamespaceTable().getIndex(namespaceUri). For UA specification nodes this is “http://opcfoundation.org/UA/”. Seems the uri for namespace where MyDemoObject is is “DemoNodeManager”

The translateBrowsePathsToNodeIds is used for known paths, e.g. if you have object instance of known type that has some hierarchy you can use the translation to find e.g. a Property from the structure only by knowing the NodeId of the instance. So you need to know the path and use .getIndex(…) for each (or use the namespaceIndex directly if you know it and know that it wont change) path element (usually they all have same namespace, but standard Properties have 0 as NamespaceIndex in their BrowseName).

If you don’t know the AddressSpace of the Server, you need to browse it. If you know a NodeId beforehand, you can also use UaClient’s methods(e.g. readXXX, writeXXX) that take NodeId as a parameter