When reaching the reCAPTCHA code only exceptions were thrown and no antibot validation took place.
Uncaught ReferenceError: RecaptchaState is not defined Uncaught TypeError: Cannot call method 'addEventListener' of null Uncaught TypeError: Cannot call method 'focus' of null
After a lot of investigation and going through several git commits and finally comparing the beta version with the production code I found out that in a config file wrong credentials were used for the domain under which the beta version was running. When you create your public and private key in your reCAPTCHA account you have to specify a domain for which the keys are valid.
In my case I must have edited the configuration file or copied it from the production code such that the used keys just became invalid with the domain of the test system.
Before you’re driving crazy, check your keys.