I also had the same problem after upgrading from 16.04 to 18.04.
-- system below --
Ubuntu 18.04.2 LTS
deja-dup 37.0
duplicity 0.7.17
-- error message below --
GPGError: GPG Failed, see log below:
===== Begin GnuPG log =====
gpg: WARNING: "--no-use-agent" is an obsolete option - it has no effect
gpg: decrypt_message failed: Unknown system error
===== End GnuPG log =====
I also had the same problem after upgrading from 16.04 to 18.04.
-- system below --
Ubuntu 18.04.2 LTS
deja-dup 37.0
duplicity 0.7.17
-- error message below --
GPGError: GPG Failed, see log below:
===== Begin GnuPG log =====
gpg: WARNING: "--no-use-agent" is an obsolete option - it has no effect
gpg: decrypt_message failed: Unknown system error
===== End GnuPG log =====