Hi,
I just solved a very annoying problem.
Three of my calendars stopped showing events after a certain date in the past.
I tried to unsubscribe and delete these calendars, but they kept popping up time and again.
I could not find an answer to the problem. So, this helped me.
In the end I deleted all calendars and those three kept still popping up.
I guess the solution was to edit the file pref.js and remove the entries of those three calendars.
In the process I also emptied the directory of /calendar-data/.
Now I am linking up all the calendars again.
Hope it helps someone else one day.