The misleading error message is a bug, there should be a more coherent error message there.
Ironically, I "broke" this when adding a workaround for HTC's "how can we avoid the Apple patent on opening links" code crashing...
Back to .ICS - the app is trying to open it as any other attachment type, and stock Android Calendar doesn't support these.
There are apps in Google Play that can open .ics's, and act as a bridge to the device's Calendar.
Cleaning this mess and making it possible to work with calendar invites right from the app is on my list for the next release / update.
