I have a user that had a very old and very large public keyring.
He encountered errors (gpg: keydb_get_keyblock failed: Value not found) during
migration to gnupg 2.1 and was unable afterwards to decrypt things.

Through some testing / bisecting (and realizing he had a weird key in his pubring):
pub 999R/F88341D9 1994-11-08

I came down with this boiled down script to reproduce the problem (using gpg
1.4.16 and gpg2 2.1.7-beta3):

  export GNUPGHOME=$(mktemp -d)
  gpg --keyserver hkp://keys.gnupg.net --recv-key F88341D9
  echo Key-Type: RSA >> $GNUPGHOME/test
  echo Key-Length: 1024 >> $GNUPGHOME/test
  echo Passphrase: 123 >> $GNUPGHOME/test
  echo Name-Real: PGP2 is annoying >> $GNUPGHOME/test
  echo Name-Email: bugs@g10code.com >> $GNUPGHOME/test
  gpg --gen-key --batch < $GNUPGHOME/test
  gpg2 -K

Result:

gpg: NOTE: THIS IS A DEVELOPMENT VERSION!
gpg: It is only intended for test purposes and should NOT be
gpg: used in a production environment or with production keys!
gpg: starting migration from earlier GnuPG versions
gpg: porting secret keys from '/tmp/tmp.ucwuHgbgNt/secring.gpg' to gpg-agent
gpg: keydb_get_keyblock failed: Value not found
gpg: key 805110EB: secret key imported
gpg: migration succeeded
gpg: checking the trustdb
gpg: keyring_get_keyblock failed: Unknown system error
gpg: failed to rebuild keyring cache: Unknown system error
gpg: 3 marginal(s) needed, 1 complete(s) needed, PGP trust model
gpg: keydb_search_first failed: Legacy key
gpg: validate_key_list failed

/tmp/tmp.ucwuHgbgNt/pubring.gpg

sec rsa1024/805110EB 2015-07-03
uid [ultimate] PGP2 is annoying <bugs@g10code.com>

sec rsa1024/805110EB 2015-07-03
uid [ultimate] PGP2 is annoying <bugs@g10code.com>