Hello @takayukister . I just wanted to echo what several others have reported with version 5.4. Our agency has approximately 500 sites running contact form 7 and have for many years. Around 40 of them were updated to 5.4 and began experiencing intermittent submission issues as described above. As @c4concepts pointed out, clearing the cache in our case also resolves the problem temporarily for those 40 sites but returns within 24 hours (sites running on wp engine with wp rocket and Cloudflare). We have rolled back those sites to previous versions of contact form 7 and everything is submitting okay as of now. Will hold off for a patch or communication before updating again.
