Hey Steven. Thanks for posting; and for confirming (via email) that resaving your permalinks didn’t do the trick. What’s weird is that when I test this exact same code on my default 2011 install no error occurs and that file downloads without issue.
A couple thoughts on this. First, have you reviewed this thread (https://theeventscalendar.com/support/forums/topic/webcal-subscribe-problems/) which seems to address this issue at one point? It’d be later on in the thread, around late February. See if you share any similarities with those guys?
I also know that our dev Joey is looking into this now. After reviewing the issue in that other thread we have come to a consensus that it’d be wise to review and rewrite part of the iCal function whilst taking into account the latest Google Calendar API documentation (http://code.google.com/apis/calendar/publish/). We’ll be doing that for the 2.1 release.
Sorry I couldn’t offer up more here. Let me know after you’ve had a chance to digest the points above.