found it out, setTransient(true) must be set to load the state from the server
Dani
// Invalidate and rebuild tree
this.treeModel = null;
// this.htmlTree = new HtmlTree();
this.htmlTree = null;
this.selectedNode = null;
// getTreeData rebuilds the tree from scratch
getTreeData();
String[] expandedPath = { "0" };
TreeStateBase treeState = new TreeStateBase();
treeState.setTransient(true);
// Expand the tree“s first level nodes
treeState.expandPath(expandedPath);
treeModel.setTreeState(treeState);