Yes, I'm sure, because I had to entry the URL manually in the plugin code and the calendar is still public.
But the plugin ignores my changes, and even on a local installation of wordpress on my PC at home, the plugin tells, that the ICS is not updateable, although I didn't configure one at all!
In fact, I was fronted with an irreasoned error message. Just by activate the plugin, the error comes up. I did really nothing but just activate it directly after installation! That have to be all, if there are no installation instructions...
I have written my own ICS plugin now, here is the code:
<?php
echo "Your ICS file cannot be updated.";
?>
...it's pretty much smaller and has the same effect than the original! :)