GnuPG: there is no assurance this key belongs to the named user

Beim Einrichten von Schlüsseln für das Backup Script duplicity hat mich folgenden Fehlermeldung ereilt

==============================================================================
GPGError: GPG Failed, see log below:
===== Begin GnuPG log =====
gpg: D7E81F5B: There is no assurance this key belongs to the named user
gpg: [stdin]: sign+encrypt failed: unusable public key
===== End GnuPG log =====

Dies ist bei mir aufgetreten, weil ich einen Schlüssel von einem anderen Server importiert habe. Der Schlüssel wurde zwar importiert aber "ihm wurde noch kein Vertrauen ausgesprochen". An dieser Stelle habe ich die Lösung gefunden.

gpg --edit-key xxxxxx
gpg (GnuPG) 1.4.10; Copyright (C) 2008 Free Software Foundation, Inc.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

Secret key is available.

pub  4096R/xxxx  created: 2013-07-18  expires: 2015-07-18  usage:
                     trust: unknown       validity: unknown
sub  4096R/xxxxx  created: 2013-07-18  expires: 2015-07-18  usage: E   
[ unknown] (1). Hagen Bauer (Duplicity Key Backup Signatur) <Hagen.Bauer@web.de>

gpg> trust
pub  4096R/xxxx  created: 2013-07-18  expires: 2015-07-18  usage: SC  
                     trust: unknown       validity: unknown
sub  4096R/xxxxxx  created: 2013-07-18  expires: 2015-07-18  usage: E   
[ unknown] (1). Hagen Bauer (Duplicity Key Backup Signatur) <Hagen.Bauer@web.de>

Please decide how far you trust this user to correctly verify other users' keys
(by looking at passports, checking fingerprints from different sources, etc.)

  1 = I don't know or won't say
  2 = I do NOT trust
  3 = I trust marginally
  4 = I trust fully
  5 = I trust ultimately
  m = back to the main menu

Your decision? 5
Do you really want to set this key to ultimate trust? (y/N) y

pub  4096R/xxxx  created: 2013-07-18  expires: 2015-07-18  usage: SC  
                     trust: ultimate      validity: unknown
sub  4096R/xxxxx  created: 2013-07-18  expires: 2015-07-18  usage: E   
[ unknown] (1). Hagen Bauer (Duplicity Key Backup Signatur) <Hagen.Bauer@web.de>
Please note that the shown key validity is not necessarily correct
unless you restart the program.

gpg> save

Key not changed so no update needed.

Dann klappte es