Are you able to login by SFTP?
The reason I ask is because when you get web server error messages like that they appear in the file /home/username/logs/[your domain]/http/error.log
So that's the first thing you want to check when you get mysterious 403, 404, 500 or 503 error messages. 403 errors usually mean there is a problem with permssions (eg you have files with no public read permission) or ownership (contact support if your stuff belongs to any user but the one you SFTP as, esp for 'root' or 'staff') but they could also be caused by directives in .htaccess files (eg block by IP or referrer, no generated indexes etc) or even output from CGI scripts/web apps.
So what does your error.log say??
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