My message is written in French (Une erreur critique est survenue sur votre site. Veuillez consulter la boite de réception de l’e-mail d’administration de votre site pour plus d’informations.), but I’m pretty sure that it is the translation of the error message in the first picture of the link you sent (https://webnus.net/dox/modern-events-calendar/what-to-do-if-i-see-fatal-error/).
I checked the version of the plugin installed and it is the 5.5.0 (the lastest).
