Version:           3.5.3 (using KDE 3.5.4, Debian Package 4:3.5.4-2 (testing/unstable))
Compiler:          Target: i486-linux-gnu
OS:                Linux (i686) release 2.6.16.20lincoln

The only way I can think to explain this bug is through an example:

1. Set up a WebDAV server, if you don't already have one.  Publish an iCalendar file on the WebDAV server (shouldn't matter what's in the file--mine was empty).

2. Set up KOrganizer to use the iCalendar you just published.  It may or may not be read-only; it doesn't seem to matter.  But do make sure that it will be automatically reloaded on a regular interval (otherwise you won't be able to reproduce the bug).

3. After verifying that you can read the remote calendar via KOrganizer, remove the calendar from KOrganizer.  Then (and only then) delete the iCalendar file from the WebDAV server.

Depending on what interval you set for automatic reload of the remote calendar, you should soon see a "file not found" error when it tries to reload the calendar from the WebDAV server--despite the fact that you removed it from KOrganizer!