Modern Events Calendar still works with X and PRO, It was no longer available on CodeCanyon so the developer (WEBNUS) now is selling it directly on his site, we will continue to provide MEC as part of our extensions.
Extension - Modern Event Calendar
Please clarify the issue and how we can replicate it, so we can have a better idea.
You mentioned above that this issue happened after you updated X, remember to clear all caches (plugin, server-side, CDN, and browser's cache) after updating so that the code from the latest release is always in use. This will help you to avoid any potential errors.