In connection with a PHP migration to 7.1 or 7.2, the All-in-One Event Calendar may stop working with the following error message: CSS compilation failed because not enough free memory was available (at least 24M is required). Your calendar will not display or work properly without CSS.
Solution: First, check if there really is a problem with the memory limit of the server. If this is the case, you can create a php.ini file to increase the limit (see Server & Hosters for more information).
If this problem is solved, one should click in the plugin itself under „Events“ > „Theme Options“, on „Save Options“, clear the website cache (if available) and the browser cache.