In the ACP Captcha configuration page after switch to the properly configured reCaptcha an javascript error appear.

The solution is, to add in the file adm/style/captcha_recaptcha.html after:
lang : '

',

the line:
tabindex : <!-- IF $CAPTCHA_TAB_INDEX ->{$CAPTCHA_TAB_INDEX}<!- ELSE ->10<!- ENDIF -->

or delete the comma after the lang definition.