Our guess is that "das mithaftende Grundbuch" refers to the institution or person sharing liability and not the Grundbuch itself, i.e. sth. like "die mithaftende Grundbucheintragung". As this hunch is based merely on intuitive interpretation but no hands-on evidence, our confidence level is at guessing level. Perhaps someone else can corroborate with proof if he/she feels this may be going in the right direction. HIH