Accept the fact you couldn't fix the issue and handle the questions and simply blame SIGT for the mistake on your side. Their DB is "tested with up to one billion 100-byte entries, i.e., ~100GB". Read it for yourself https://github.com/signatumd/source/blob/master/src/leveldb/NEWS. And FYI, Bitcoin and Litecoin uses the same DB. What reason are you going to come up with now?