Either create a new contact entry with a mail form or activate user registration in the user options.
Leave the fields empty and submit either form.
Client-side form validation should kick in and prevent the form from being submitted.
The submission of the form(s) is intercepted by the browsers interpretion of the validation instead. If you add the "novalidate" attribute to the form tag in the HTML Markup (Browsser: F12) and submit them again, the client-side validation won't prevent the form from being submitted (.preventDefault()).
According to my research, the problem was first mentioned in the forum by HSorgYves in mid-February this year. While his proposed solution from one of those posts seems to fix one "symptom", it has some side effects in other areas (e.g. backend login, edit forms, aso).
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