I am using the latest Invision Community Version 4.4.10 and V3 Invisible Captcha protection for signing up new members. 

During a recent sign-up test, I discovered that although all credentials are provided correctly, the message "You did not pass the security check. Please try again." is triggered. 
The Captcha symbol is shown in the lower right corner of the screen, it seems to load correctly. 
The registration can be completed by just pressing the "Sign-Me-Up button" a second time. 

I've tested this on desktops (Chrome, Opera) and on iOS (Safari), the behavior was identical. 

The site and secret keys are correct, Cache has been cleared on browser and Forum-Level. I've also created a fresh Invisible reCaptcha with Google, so far it did not help either. 
So I am guessing the could be a bug somewhere. Anyone able to test their registrations using the same registration setup (V3 Invisible reCaptcha)?
 

Ok, just learned in another thread that Invision Community is still using V2 reCAPTCHA (Invisible and normal). So this might be the reason why I encountered the above issue. 
However, after creating and using a new Invisible reCAPTCHA in Google and updating the corresponding keys in ACP, I still get the above error. Perhaps it will take some time until the new key becomes effective and I will check back in a few hours. 

Update to this issue: I discovered that Registration/Login Methods (Facebook and Google) caused this behavior. Affected is only the pop-up registration form. Standard form works. 
Once I disabled the additional methods, it worked. Contacted Support, perhaps a bug. 

Thanks for sharing.  We ran into an issue with a customer not being able to register due to inexplicably failing the security check.  Whitelisted their email and IP to no avail.  Registered them manually then tried to reproduce it, but failed.

I also wrote a plugin that adds the "Title" field to the Contact Us form, but the additional field sometimes causes captcha to fail as well, so I had to disable the plugin.

Captcha is flaky in general in my experience, unfortunately.

Thanks for sharing your experience. With the help of support I was finally able to locate the "troublemaker". It was no bug caused by Invision Community, but a server configuration issue. 
I had the PHP modules suhosin7.so and acpu.so in php.ini activated. When checking the PHP error log, it showed that suhosin7.so could not be found. I discovered that suhosin7.so is running on PHP 7 and 7.1 only, but not on 7.2 (the one I use). After removing the two modules from php.ini, all worked well.