If you use a cache plugin on your site, you may receive a 403 error on Ajax queries. More specifically, when you clicked the button to launch the popup or submit the form, you received an object object alert.
It could be due to the presence of a nonce, which is used to validate requests. Setting the Clear Cache Lifespan to 10 hours or less should fix this.
For more information, please refer to the documentation for your cache plugin.