Website builder "No active user session found" error

Website builder cannot be accessed directly by URL, for example: http://builder.mysite.com. It can be opened only via public demo URL (that you can find you your brand) or via website builder plugin installed on hosting panel;
In case error URL of page contains part similar to "?login_hash=a1b2c3...", it means that you have wrong website builder plugin installed on your hosting or you installed it before website builder installation. You need to delete it and install plugin anew using plugin installation guide link from On-Premises brand.
Also, please check website builder files owner and group. If you extracted website builder archive from SSH terminal being logged in as root, you need to chown files to a correct user and group.