Please login to your backend, then navigate to Components/Improved AJAX Login & Register/Form manager. Then click on the current form to edit. In the form manager, click on the captcha input field, and remove the validation pattern on the right. This should solve the issue.

Description: Google changed the captcha validation, so now captcha also can contain one or two words despite of the previous version, what only contains 2 words. That's why you need to remove the pattern validation of the captcha input field, because that pattern now contains validation for the 2 words version.