verumgero,
I've found that uninstalling via Application Manager does not remove the mcalendar data files. You may need to do this manually. I'm not a very experienced hand at Linux, but I was able to do this using Midnight Commander. I usually use emelfm2, but the icons are pretty cryptic in emelfm2 and I couldn't figure out how to do a search for "mcalendar*.*" in emelfm2. Also, when I tried the "find" command in X-Terminal, I kept getting the "too many files/folders opened error".
In Midnight Commander (aka MC), I was able to find the mcalendar pref file and database file (after running find "mcalendar" and then stumbling around a bit) and delete them. After that, a reinstallation of mcalendar worked fine.
As an aside, I had to uninstall Midnight Commander because my n810 was rebooting shortly after anytime I connected to the internet. I had installed the mileage app and Midori at the same time as MC, so I'm not sure if those were problems too. Before I uninstalled MC, I ran the hometools applet to see what happened to cause the reboots. I noticed that, whenever I connected wirelessly, my Total Memory would drop quickly from about 70000 k to 17000 k before the tablet rebooted. I've not had this problem since I uninstalled MC and the mileage app. (I had previously uninstalled Midori by itself, but still had the problem until I also uninstalled mileage and Midnight Commander).