Univention Bugzilla – Bug 33162
login screen does not appear if display was not attached during boot
Last modified: 2013-12-17 11:12:42 CET
A black screen (the one before starting lightdm greeter) or the UCC (green) image of bootsplash will appear later when connecting a display in a UCC client.
All processes and policies will run properly though.
A restart of lightdm will "fix it"
service lightdm restart
It might be related with Bug 30402
The Wayland protocol will provide monitor hotplugging based on KMS, but that's rather something for the future.