This is a bit of a wild guess, but it may have to do with the permission settings in your course / Moodle installation. In order to add slots, or indeed to see the admin screen, you need to have the capability "mod/scheduler:manage". As a teacher or administrator, you would normally have these. However, maybe some permission settings were changed in that specific course?
Scheduler (plugin)
baffled by Scheduler (again)
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