If the .service file for the current display manager is removed, the DM cannot be changed cleanly. [root@kwpolska-lin system]# systemctl enable lxdm.service Failed to issue method call: File exists [root@kwpolska-lin system]# systemctl disable kdm.service Failed to issue method call: No such file or directory The only fix is to manually remove /etc/systemd/system/display-manager.service and link it to the new DM, or temporarily rename a DM .service file to the one that got removed. (I found out about the bug after getting rid of Plymouth, using kdm-plymouth.service instead of kdm.service. Plymouth sucks under Arch Linux.)
You can use "systemctl enable --force". That said, your packaging scripts are broken. The old display manager should have removed the symlink on removal. This is usually done from the package removal scripts of your display manager rpm. Please report this to your distribution, to the package of your old DM.
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