Few points here.

• We use ajax based validation which does not require user to enter anything. There is some sophisticated logic behind that.

As for captche internal logic there are two options.

1. Do not use session cookies for captcha validation, this allows to have instance on one domain and embed code in n+1 domain without any issues. This is the default captcha logic.
2. Use session cookies "Chat configuration" > "Use session captcha. LHC have to be installed on same domain or subdomain." just set 1. Drawback of this method is that, chat won't work correctly if you have chat installed on one domain and using it on different domains. This option to work correctly requires chat to be installed on the same domain/subdomain othwerwise users will experience "Invalid captcha code, please enable Javascript!" error messages.

In general I recommend to use second option if you have chat installed on the same domain and use the first one if you have installed chat on different domain.