Hi Takayuki,
I already checked those links on the FAQ before writing.
All of those scenarios do not seem to apply because the forms were working perfectly and stopped working without me doing anything at all.
Also, the form with the spinning arrow icon, are still sent and arrive properly, despite not getting the confirmation message on the form, while the message with the red border alert is blocked.
So if I didn't install any new plugins after the forms were active and working, and I change no settings, I'm not sure why it should break down without me doing anything, but suspiciously only after the new version of WP was installed & activated?
Thanks for the ongoing support! :)