Anyone else seen the issue with the screensaver getting stuck? Moving the mouse or hitting any keys will not return to the login window or desktop. We have to hold the power button down until it goes to sleep and hit the button again to bring it back.
We already reset the PRAM too.
@Caist have you tried with no config profiles installed? There were numerous conflicts with Security & Privacy, Restrictions, and Login Window config profiles that can lead to this exact issue.
We too currently have issues running screensavers over the loginwindow (in our case it involved the "Classic" saver and pictures). Alike you, the issue does not exist when a user is logged in.
In my experience and testing, I have noticed is this issue effects 10.12.x only, as 10.11.x and earlier have had no problem. The High Sierra Public Beta's also seem to work as expected.
I have had bug report filed for a couple months. Report #32595997
@unserializedMLB In my case, the issue is called by MediaLibraryService crashing.
Custom Photos using the iLifeSlideShows Screensaver (Classic Screensaver) over loginwindow are run via a "system" user. This user has a euid lower than the NSMinimumUserID.
The bug extends from this... The caller will get a nil MLMediaLibrary and MediaLibraryService will not launch.
Though this worked in prior OS's and Apple claims it now works in 10.13.x (though I have not personally tested), 10.12.x fell victem to this big with no resolution from Apple. My Bug Problem ID is 32595997
Last update via Apple:
We believe this crashing issue is resolved.
Please test this issue with the latest macOS 10.13 and update your bug report at https://bugreport.apple.com/ with your results.
Hope that helps, at least a little...