i have the same problem. When i clear all cache , not stop the WP Fastest Cache , then Contact Form 7 work well , but one day later , Contact Form 7 can not work. This condition i test for 3 days , every day is this condition. I clear cache in WP Fastest Cache , it work well , but about 1 day later , have new Cache in WP fASTEST cACHE , it not work.
If the version 5.3.2 can still work and safe ? If still work and safe , maybe we can still use it bedore this verison 5.4 problem be solved.
We have big pressure every day , because we can not recieve email and inquiry from our customer. Hope Contact Form 7 plugin author can solve this problem