Hi Marc (or Marc’s dev as it may be). This is an odd issue that we haven’t much experience with; I can get a dev to take a look, but can you confirm whether you have any comment-related plugins active that could be conflicting here?
Right off the bat, I know that depending on your theme, using the different templating options available under Settings -> The Events Calendar might get the comment box to appear/hide.
Lastly: I saw you had some templating issues in a separate thread, which you seemed to be able to figure out on your end. Any chance this was resolved by one of those fixes?