Have exactly the same issue across multiple sites – it’s intermittent but worse now. Try and login, popup appears, put correct credentials, just get spinner in popup – never stops. In console – it says error 400 for admin-ajax. Solution is to reload or refresh browser and then click login again – works after reload no problem every time. This is impossible for users (customers) – can’t tell them to reload the page. Have tried switching off all other security plugins — am on the latest versions of WordPress / Woocommerce.