sddm package (not installed) failed to install / upgrade: subprocess installed post-removal script returned error status 1

Once installed in my PC containing also LIGHTDM and GDM, the first doesn't have a sddm.conf file and it take a long time to login me. The second one doesn't appear. Only the third is working. So I'm trying to uninstall SDDM in order to see if LIGHTDM works again (it looks me more than the others).

ProblemType: Package
DistroRelease: Ubuntu 4.15
Package: sddm (not installed)
Uname: 4.0.2-040002-generic Linux x86_64
ApportVersion: 2.17.2-0ubuntu1.1
AptOrdering:
  sddm: Purge
  NULL: ConfigurePending
Architecture: amd64
Date: Wed Jun 24 15:06:42 2015
DuplicateSignature: package: sddm: (not installed): subprocess installed post-removal script returned error status 1
ErrorMessage: subprocess installed post-removal script returned error status 1
InstallationDate: Installed on 29/04/2015 (55 days ago)
InstallationMedia: Ubuntu 4.15 "Vivid Vervet" - Release amd64 (20150422)
RelatedPackageVersions:
  dpkg 1.17.25ubuntu1
  apt 1.0.9.7ubuntu4.1
SourcePackage: sddm
Title: sddm package (not installed) failed to install / upgrade: subprocess installed post-removal script returned error status 1
UpgradeStatus: No upgrade log present (probably fresh install)