Overview
When the Form Submission step is assigned to an email assignee, a warning may display when the assignee accesses the submission link from email: “The link to this form is no longer valid.”
This issue occurs when the submission page is being cached. Exclude the submission page from your caching configuration to resolve the error.
Some hosting services, such as WP Engine, cache aggressively. If you are unable to exclude the page yourself, please contact your hosting provider to request that they remove the page from their cache.
Refer to the FAQ on Cache and Script Optimizer Issues article for more information.