Here is the stacktrace from the log:

INFO: Captcha validation had a problem
com.octo.captcha.service.CaptchaServiceException: Invalid ID, could not
validate unexisting or already validated captcha
at com.octo.captcha.service.AbstractCaptchaService.validateResponseForID
(AbstractCaptchaService.java:138)
at
com.octo.captcha.service.AbstractManageableCaptchaService.validateResponseForID
(AbstractManageableCaptchaService.java:365)
at hudson.security.SecurityRealm.validateCaptcha(SecurityRealm.java:232)
at hudson.security.HudsonPrivateSecurityRealm.createAccount
(HudsonPrivateSecurityRealm.java:154)
at hudson.security.HudsonPrivateSecurityRealm.doCreateAccount
(HudsonPrivateSecurityRealm.java:118)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at org.kohsuke.stapler.Function$InstanceFunction.invoke
(Function.java:156)
at org.kohsuke.stapler.Function.bindAndInvoke(Function.java:76)
at org.kohsuke.stapler.MetaClass$1.doDispatch(MetaClass.java:73)
at org.kohsuke.stapler.NameBasedDispatcher.dispatch
(NameBasedDispatcher.java:30)
at org.kohsuke.stapler.Stapler.invoke(Stapler.java:438)
at org.kohsuke.stapler.MetaClass$4.doDispatch(MetaClass.java:145)
at org.kohsuke.stapler.NameBasedDispatcher.dispatch
(NameBasedDispatcher.java:30)
at org.kohsuke.stapler.Stapler.invoke(Stapler.java:438)
at org.kohsuke.stapler.Stapler.invoke(Stapler.java:356)
at org.kohsuke.stapler.Stapler.service(Stapler.java:116)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:802)