What happened to CUPS on localhost:631?

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cups.service - CUPS Scheduler
   Loaded: loaded (/lib/systemd/system/cups.service; enabled; vendor preset: enabled)
   Active: active (running) since Sat 2017-05-06 09:26:27 CEST; 1min 14s ago
     Docs: man:cupsd(8)
 Main PID: 4404 (cupsd)
   CGroup: /system.slice/cups.service
           ├─4404 /usr/sbin/cupsd -l
           ├─4429 /usr/lib/cups/notifier/dbus dbus:// 
           ├─4430 /usr/lib/cups/notifier/dbus dbus:// 
           └─4431 /usr/lib/cups/notifier/dbus dbus:// 

May 06 09:26:27 faustino systemd[1]: Stopped CUPS Scheduler.
May 06 09:26:27 faustino systemd[1]: Started CUPS Scheduler.

And this is weird: I refreshed the localhost:631/admin page and this time it worked! But later on refreshing got an error again as did refreshing the localhost:631 page.