Conflict with Captcha plugin

I’m using this captcha plugin:
http://ww.wp.xz.cn/plugins/captcha/

When someone wants to fill in the captcha when leaving a reply, he/she always gets the message the captcha wasn’t right. Deactivating quicke cache solves this.

The only setting I have in quick cache is the option ‘enabled’, the rest is off.

https://ww.wp.xz.cn/plugins/quick-cache/