Mailster form uses WordPress Nonces to prevent invalid form submissions. These nonces can expire in some cases which causes an error message if you like to signup to a newsletter.
If you see this message it’s most likely that your page has been cached and the nonce is expired. By default the expiration time is 24 hours.
Caching plugins offer an expiration time for their cache and if you cache pages longer than 24 hours the nonce will expire.
If you are not able to disable your caching settings you can disable caching on pages with Mailster forms. Visit the Advanced Settings page at Newsletter => Settings => Advanced and check the disable form cache checkbox.
Clear your page cache and test your form again.