A recent pull request requires 'isCrossRef: true' for the many to many list widgets to be generated in forms. Pull Request: https://github.com/propelorm/sfPropelORMPlugin/pull/90
The default forms will throw a fatal error when they try to set the labels for these list that aren't there anymore in the base classes.
Posting this up on SO in case someone else runs into this problem, because it took me a while to figure out.
this is fixed since a week, see: https://github.com/propelorm/sfPropelORMPlugin/pull/136