Getting 403 code: “rest_cookie_invalid_nonce”

Dear support,
a website managed by me is getting this error:
{code: “rest_cookie_invalid_nonce”, message: “Il nonce del cookie non valido”, data: {status: 403}}
code: “rest_cookie_invalid_nonce”
data: {status: 403}
message: “Il nonce del cookie non valido”

User can see the spinning wheel rotate forever, emails by the way are sent both admin that customers.

I checked all google and previous problems here on the support page, I’ve checked all the syntax and looks good.
The form is hidden in a private area for registered users, so you need to login before trying to submit.
Is there any change to give you user/passw to have a look?

any suggestion is appreciated.
Thank you