We will get this fixed in upcoming update. Until then, can you pls modify line number 869 of framework/functions/utility-functions.php file to
$args['image_size'] = 'full'; // retain original image - don't bother cropping
Or you can copy this function mo_get_thumbnail_args_for_singular() with the above change to your child theme as well if you do not want to modify the original file.
This change will be part of next update and hence no need to worry about things being overwritten.
Any particular reason you are using All in One Events Calendar and not the “The Events Calendar” plugin? Featured image seems to work fine with the later plugin.