I just came across the fact that listBibl is not valid as a child <back>. There is a bug report on this on SF https://sourceforge.net/p/tei/bugs/532/ but it seem that the fix misses <listBibl> since it is not part of model.listLike.
Mag. Daniel Schopper
Austrian Centre for Digital Humanities
Working Group Data, Resources and Standards
Austrian Academy of Sciences
+43 1 51581-2332