Hi threecedars.
When I view the resulting .ics file I can see that it is being modified by WP Super Cache which is appending an HTML comment to the bottom of the file. Of course, it isn’t an HTML file at all and that is quite likely to be causing this problem.
With that in mind, can you try removing WP Super Cache (follow the vendor’s advice on doing so closely, as merely deactivating it does not always guarantee its effects will cease) and see if that remedies the issue?
Thanks!