There’s been numerous bug reports regarding Event Organiser (and the Pro extension) after updating to WordPress 5.5. The plug-in has been tested against WordPress 5.5 and so should work; all bug reports I have seen thus far have been traced back to other plug-ins which broke with the update.
In WordPress 5.5, WordPress updated the jQuery version it used which removed some deprecated functions (most notably $.live and $.browser). Any plug-in using these functions will cause an error on the page and will likely break the functionality on that page (e.g. the calendar, or the booking form).