What does a 400 error mean when our form submits?

if it only happens for some people, try it in a fresh incognito window. nginx returns a 400 when the cookies get too big ("request header or cookie too large"), usually from piles of tracking or chat-widget cookies. clearing cookies fixing it is the giveaway.

the server got the submission and refused it as malformed. look at the response in dev tools (Network tab, click the failed request, Response), it usually says why. commonest cause I see: a security token in the form that's expired because the page was cached. works for you logged in, fails for visitors. if it's WordPress with a caching plugin, exclude the form page from the cache and test again.