looks like you changed your PHP settings, or your hosting provider did it. Please check your settings and deactivate “PHP Errors and Warnings”, this is only for developers and very dangerous on a public website. The PHP settings are also the reason why you cannot log in to your website, the output buffer is not set correctly. As soon as you change the PHP settings you can use your website again without any problem.
Please make also sure your theme is up to date, all current releases of our themes don’t have any warning.