Just did a quick look at a site that is using acf_form and I find this in the generated form
<input type="hidden" id="_acf_nonce" name="_acf_nonce" value="17630f97db">
I do not know what is causing this on your site. I would start by seeing if there is some type of plugin conflict by deactivating other plugins. Also not the difference in the field name, on my site it starts with an underscore (_)
