new user registration fails with reCaptcha error - missing reCaptcha settings under admin section

Hello

I've been running Gitlab CE as Docker instance for over a year now and there were no issues so far. My last user registered about 5 month ago without issue and I had a new user to register yesterday but he couldn't.

After filling out the form and successfully solve reCaptcha, the site takes quite a while to load an then fails with error messsage
There was an error with the reCAPTCHA. Please solve the reCAPTCHA again.

I tried to "setup" reCaptcha with that guide https://docs.gitlab.com/ee/integration/recaptcha.html , but i cant even find a single hint of "captcha" in my admin settings as described.
There isn't even a checkbox to enable/disable captcha so I cant disable it at least for now.

Which this issues I'm kind of stuck... I dont know where to look at.

• instance is alive since gitlab 9 (probably) has been updated weekly since then
• Gitlab CE (Docker edition) - 11.5.3 (gitlab-ce@aa117376d89f0146cc7c573ec9091ada6041b417)
• registration worked 5 month ago - broken now
• config wasnt changed from my side
• no "recpatcha" settings under Admin > Settings > General
• checked gitlab.rb file but also no sign of "captcha"
• not similar to gitlab-ce#45852 - not error in production.log

If you need any more information I'd be happy to provide it