As reporting in the mailing list, the on-demand retention strategy is currently
completely broken.
(http://www.nabble.com/Slave-Agent-Terminates-with-Stacktrace-td23734729.html#a23734729)
A patch has been submitted on the mailing list, but no response has been made.
The patch should either be applied and fixed, or the feature should be removed
until the feature is in a usable state.