Frontend form not working ("Oops...") when on server

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I worked more on this, but did not succeed.
I would appreciate help.

I found out that the “oops” error is related to the “NONCE_NOT_VALIDATED” error from the form plugin. But I only saw it in the translations from the languages.yaml in the forms.
by grepping around, I also found that error in the plugins/form/classes/Form.php file.

        if (!$this->values->get('form-nonce') || !Utils::verifyNonce($this->values->get('form-nonce'), 'form')) {
            $this->status = 'error';
            $event = new Event(['form' => $this,
                'message' => $grav['language']->translate('PLUGIN_FORM.NONCE_NOT_VALIDATED')
            ]);
            $grav->fireEvent('onFormValidationError', $event);

            return;
        }

So I reckon this checks for a “nonce”.
And indeed, when I comment out the block above, my form submits smoothly.

Could someone please explain what a nonce is and why I do not get it on a website with default configuration?

Thank you in advance!

PS: I saw this, and I am using firefox. Maybe someone could resolve that other issue?