This is a well-documented bug. . . you have OpenSSH installed, don't you?
When calendar opens, it scans all of the files in the home directory (/home/zaurus on the Sharp ROM, IIRC). The OpenSSH files in your home directory are readable only by root. Calendar tries to read these files, can't, and crashes.
The solution is to change the permissions on the OpenSSH folder in your home directory to be world-readable (chmod 711 OpenSSH), or move them to another directory.