subscribe without commenting

Hi, I’ve recently been getting a new error on my site. When subscribing to a post without commenting, I’m directed to a page with the error “You may not access this page without a valid key.”

This error was not occurring previously, and I can’t resolve it, especially with my limited technical knowledge. Please help, thanks!

https://ww.wp.xz.cn/plugins/subscribe-to-double-opt-in-comments/