I have the same issue in 18.04.
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https://bugs.launchpad.net/bugs/1771880
Title:
Seahorse unable to import certificates in Ubuntu 18.04
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Public bug reported:
Update failed.
ProblemType: Package
DistroRelease: Ubuntu 16.04
Package: cups-daemon 2.1.3-4
ProcVersionSignature: Ubuntu 4.4.0-66.87-generic 4.4.44
Uname: Linux 4.4.0-66-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.5
Architecture: amd64
CupsErrorLog:
Date: Mon Mar 27