Found the following info from Microsoft:
Here, Microsoft states:
"Calling a pure virtual function is a programming error, so you need to find the call to the pure virtual function and rewrite the code so it is not called."
I've recently installed WP 2.0, and tried to open my site in IE6 which caused the problem.
I've used the same modifications to my WP 2.0 installation as I did to a testsite running WP 2 RC2. There this problem doesn't occur.
Problem is I haven't rewritten any functions, merely done some cosmetical changes, and enabled the EC3 calendar plugin (no, disabling it doesn't help).