Crazy iCal - doesn't show any events and to-dos

Question

My iCal is going crazy. It all started with an inspector popup not closing. I clicked on "Done", but it just stayed there. I switched to a different week, it stayed at its position. I closed the window, it disappeared, but when I reopened the popup it was here again.
Then I restarted iCal and it was finally gone. But then even more strange things happened. I was doing some more "intensive" iCal-work. I exported/imported two calendars to merge them with others, I switched some events to different calendars, I edited some events, and did some copy/paste from repeating events (there was a warning message that only a single event will be copied). At some point there was again a strange behavior, I cut/pasted an event, but pasting didn't work. Then I pressd Command-Z to undo the cut, the event reappeared, I copied it and pasted it somewhere else. Then the events started to disappear. First the one I was editing. It was just gone. I switched to the previous week and back, and then no events were there anymore. In the other weeks there were still some (if I remember right), but when I switched to month-view - everything gone. The to-dos were still there. I restarted iCal, nothing, I restarted the Mac, nothing. Now even the to-dos are gone, although I can still see them in Mail.
It gets even worse: The left and right arrow-buttons to switch to a different day/week/month stay pressed when I click them until I click the day/week/month button. I have never seen anything like that before. I can not check/uncheck any calendar, the "today" button is greyed out. When i switch from month-view to day-view sometimes on the top you can still read half of "June 2009" from month-view.
I checked the files ~/Library/Calendars. I think the events are still there, I can view all the events (I think) in Quicklook. Also, the to-dos are still visible in Mail. It must be an iCal problem. After restarting iCal a couple of times the to-dos reappeared.
Any ideas?
[Update:]
Some events just reappered magically, among them the automatically imported birthdays from address book, but the dates are wrong. A friend whose birthday is on May 9th (that's a Saturday this year) appears now every week on Saturday!
Message was edited by: Sebastian300

Answer 1

:S,
Sorry, missed that fact. My memory is good, but short.
Try moving the "Calendar Cache" file to your Desktop. Quit iCal, and find the "Calendar Cache" file in your Macintosh HD/Users/yourusername/Library/Calendars. Move that file to your Desktop. After that, (I remember that I recommended this before), as long as you are moving files to the Desktop, repeat the plist removal procedure once again, log out/in or restart.
You should also consider removing the "To Do's" from Mail before restarting iCal. When you open iCal turn off "Show Birthdays calendar" in iCal>Preferences...>General.
Are you syncing any data? If so, what method(s) are you using?
;~)