Hello,

Yes, it's due to a wrong settings in recaptcha (could happen due to a bug fix with captcha plugin detection).

In 3.4.2 coming soon, a function will check if the settings is ok, so will prevent this issue.

Now, you can go in the Global Options.

If you want to use recaptcha, select the plugin in the "General settings" tab.
And select "show" in captcha option, on "submit an event" tab.
You may have to save again the menu item linked to the form.

There's some info about this possible issue in settings in the ChangeLog, which one is accessible in the control panel (or on liveupdate before each updates).

i had to change the place, and options for captcha, to fix an issue when a user wants to use another captcha plugin than the one included in Joomla core. (but unfortunately, i found no way of migrating the full old setting to the new good ones, to allow captcha to work better)

Hope this helps!
Cyril