I am getting the same error while editing an xref in place.
** Application reference missing: AcDbBlockRepETag, to AcDbPolyline.
As a work around, remove the editted linework from the working set then discard the changes to the xref. Open the xref rather than editting in place and copy paste the revised linework back into the xref. Its a bit of a pain but better than losing your changes. Some feedback from Autodesk would be appreciated as it seems to be a common issue.