The 'Forum' document type has confused identity in Nuxeo. It is returned as a document by the query SELECT * FROM cmis:document WHERE cmis:name = 'forum' but it's base type is reported as being cmis:folder.
As additional information, the Forum type is listed as being child of cmis:document in the CMIS Types form of CMIS Workbench but it listing its base type as being cmis:folder.
There is something amiss...