This is happening for me. I'm also not a big user so I haven't really spent time looking into it, but a couple weeks ago I was trying to show one my users from another office the Calendar and it was suddenly blank. It worked for her so I could continue the training, but I am finally getting back to looking at calendar (for certification training sessions) and it's still blank!
Was just about to create a support case and came across this thread. So I have logged into another browser (but didn't clear my cookies) and the calendar entries do show up. That is maddening What's going on Calendar? Is this related to the Winter release?
I'm still having the same issue - although when I look from a different browser it doesn't work either... I still haven't given this the time is deserves to remedy - but wish it would fix itself at some point!