400 error on Craft 3 installation When I try to install Craft CMS 3 by going to /admin the site redirects me to /index.php?p=admin/install where the Begin button shows. Clicking next send me to the screen to create an admin user. 

When I enter all the information I see an ajax call going to http://domain.com/index.php?p=admin/actions/install/validate-account but that responds with 400 Error. 

Any idea whats wrong? I have .htaccess file in place, so not sure why I see the index.php part in the url. Any help would be appreciated.
Edit: in the error log file I see those three lines

[-][-][error][yii\i18n\PhpMessageSource::loadMessages] The message file for category 'yii' does not exist: /home/rrznjzkn/public_html/craft3/craft/app/vendor/yiisoft/yii2/messages/en-US/yii.php Fallback file does not exist as well: /home/rrznjzkn/public_html/craft3/craft/app/vendor/yiisoft/yii2/messages/en/yii.php

[-][-][warning][craft\app\web\ErrorHandler::handleException] 400 - Unable to verify your data submission.

[-][-][error][yii\web\HttpException:400] exception 'yii\web\BadRequestHttpException' with message 'Unable to verify your data submission.' in /home/rrznjzkn/public_html/craft3/craft/app/vendor/yiisoft/yii2/web/Controller.php:110
You're getting that error because CSRF validation is failing.  Whether that's a bug in the Dev Preview of Craft 3 or something environmental on your end is hard to tell with the given information.  I can't reproduce it locally, though.