Hi.
As a quick solution (and not much of a proper though), you can disable Security Token check on plugin settings page: Stripe Payments -> Settings, Advanced Settings tab, look for “Disable Security Token Check” option at the bottom of the page. Enable it, then save settings. Make sure to clear your caching plugin cache if you use any.
Now it would be good to figure out WHY it’s happening. Can you provide URL to your website? If you don’t want to do this publicly, please get in touch with us via contact form.