Invalid Nonce

I have a form to register properties through the front end, it is working correctly, but today I received a video from one of the users showing me that when he clicks on register the property it shows the following message “Invalid Nonce.”, I am using version 2.1 .3 with wordpress in the latest version.

Can you tell me what could have caused this error? I googled some related things and saw that it could be because of the cache on the page, but so far we are not using any caching or security plugins, the site is in the testing phase.