419 Page Expired Message
If your participants are signing waivers on their devices, and get a 419 Page Expired message, it means they have all cookies disabled on their device.
It is pretty rare for cookies to be disabled all together, but it can happen. Typically disabling 3rd Party Cookies is adequate for stopping targeted ads and other marketing.
In order for your participant to successfully complete their waiver, they would either need to enable cookies, or depending on the browser; add "https://ewaiverpro.app" to a whitelist so cookies can be saved.
Cookies are a tiny text file saved by every modern web browser, where your preferences for websites you visit are saved - curious: read more