I'm having the same issue with one of my user too, I've tried replacing every single file inside the data folder.. it just didn't work..
I backup the whole lotus folder and put it somewhere else.. reinstalled the whole lotus note and only copy back the ID file (Mail file is in the server) to start up the program.. but the calendar just doesn't display on the "Timeslots" mode.. only the summary mode is workable..
When I tried using another PC to use the ID file to access the user's calendar it works.. the problem is only inside his own PC.. but I just can't seem to locate what thing that is causing this issue.
If anyone has any good idea on how to solve this please let me know.