when we use the environment as described above, Subversion as Branch Sources and checked "Periodically if not otherwise run" with an interval of 1 minute in the Build Triggers section, all branches get build every minute even if nothing has changed.
That happened first after we upgraded the Branch/SCM API to 2.x. With the environment Branch API Plugin 1.5, SCM API Plugin 1.3 and Multi-Branch Project Plugin 0.5.1 only those branches got built periodically, those sources changed.
I am not quite sure if this is an issue of Branch API Plugin, SCM API Plugin or Multi-Branch Project Plugin.
EDIT: This happens only if "Discard old builds" as property is chosen for Branch Sources (not for Build Configuration).
Project log says: