This is a known problem in the current version of GPE Calendar in Extras. If you have the sidebar disabled and switch to/from the agenda view it is likely to crash.

This is fixed in the Beta version of GPE Calendar. That is available in the extras-devel repository but I haven't released it to Extras because there are reports of a problem with the homepage applet (gpesummary) and I haven't had time to track them down.

Your best option is either to avoid the agenda view (many people, including me, do not use it) or try the Beta version. There are instructions on installing the Beta version at http://talk.maemo.org/showthread.php?t=22219.

Graham