Being unable to login to your WordPress site, the first step is to follow this guide.
In a client case, none of the suggested steps helped and we were still locked out. It turned out that the plugin causing the lockout (error message invalid username or password) was Invisible reCaptcha.
It is possible to enable the settings for Enable Login Form protection even though the Site and Secret Keys are empty in the Settings tab in the plugin.
After logging out, it will not be possible to login again and no error message is indicating the problem is caused by the Invisible reCaptcha plugin. This must be considered a bug.
The solution is to remove wp-content/plugins/invisiblerecaptcha using FTP, then login and reinstall the plugin and configure Site and Secret Keys.