I've updated from previous version to 2.6.1. my KNIME. The old - named: Decision Tree Learner (deprecated) - node works well, the new Decision Tree Learner node is'nt able to create decision tree view (the simple view is working..)
The message from KNIME console is: "ERROR Decision Tree Learner View instantiation failed: null"
While fixing a general bug with all views (memory leak) we introduced a new one :-( It only affects the decision tree view, which is the reason why we didn't detect it ourselves in the first place. Will be fixed in the next bug fix release.