Please make sure you don't have other reCAPTCHA plugins activated. Disable all other plugins which have reCAPTCHA. Also, when you disable it for WordPress, make sure the three options are disabled too:

it doesn't work for me. my only choice was to login via WP account in order to bypass this issue [bug] and stayed connected forever.

and this is strange because i have no issue with it on 2 other forums that i'm managing.

lucky for me, it is only on my test site.

honestly, i do not like the close integration with wp and it is better to have a stand alone captcha plug...  

We use reCAPTCHA on dozens of test and live websites. It does work and we've never faced with this issue. I recommend enable reCAPTCHA on all forms. This should work.

We'll re-check and optimize reCAPTCHA in next version to avoid such cases.

 I recommend enable reCAPTCHA on all forms. This should work.

Yes, it works but I do no want to log in to my site with captcha.

I may include it on live sites, but on test sites I prefer to enter name and pass.

So, this is a bypass solution.

We'll re-check and optimize reCAPTCHA in next version to avoid such cases.

Try to disable the last three options now (WP forms). It works fine on all our test websites.

Installed this plugin yesterday, and have the same problem as so many others. "Google reCAPTCHA data are not submitted" when I want to login as admin, but no reCAPTCHA shown.

I am not sure if there is anything else on my site using reCAPTCHA, but I tend to believe not. 

Googling for this issue reveals that wpforo is pretty much the only plugin what causes these reCAPTCHA issue. Therefore, it is unfair to pretend it the problem is related to another plugin. But this seems to be common sense for WP programmer. It's never the own product, it's always someone else's.

I am not having Akismet installed. But I will delete my reCAPTCHA keys now. Too sad.