A visitor to the website wants to join. He validates the form and sees the following message displayed :
“Form tampering detected. If you believe you’ve received this message in error please contact the site owner.”
I specify that his name, firstname and e-mail address are unknown in the database.
What does it mean and what could have happened?
The frontend form is also protected by a nonce. If you have a caching plugin, that can interfere with the validity (lifetime) of the nonce, so exclude that page from your cache or set your cache time much shorter than the nonce lifetime (which is 12 hours by default).
Or, even better: don’t use a caching plugin (I never understood how a caching plugin could cache dynamic pages …), see also: https://legacy.joshpress.net/wordpress-nonces-and-wordpress-caching/