The items.more_subfields_xml is set to random data (generated by TestBuilder), and so GetMarcBiblio does not manage to embed items (if needed).
The error is:
:1: parser error : Start tag expected, '<' not found
More precisely it explodes in C4::Items::_parse_unlinked_item_subfields_from_xml when MARC::Record->new_from_xml is called with an invalid xml
I'd recomend to add a default value (undef/NULL) for items.more_subfields_xml in TestBuilder, not to modify everywhere we build a new item.
Will be fixed by bug 17726.
*** This bug has been marked as a duplicate of bug 17726 ***