Keep getting errors and the css doesn't load

Wondering if anyone has any clues:

Failed to load resource: the server responded with a status of 500 (Internal Server Error) http://example.com/admin/admin-ajax.php?action=megamenu_css&ver=1.3.2

However, if I check

http://example.com/admin/admin-ajax.php?action=megamenu_css&ver=1.3.2 it is in fact there…

https://ww.wp.xz.cn/plugins/megamenu/