Open edX - Course about page not found on LMS and how to fix

If you click on a course from the LMS and it says "Page not found", your properly should check this variable in /edx/app/edxapp/lms.env.json and cms.env.json:

PREVIEW_LMS_BASE

It should not be the same with LMS_BASE. For example: your LMS_BASE is "yourdomain.com", so the CMS_BASE should be something like "preview.yourdomain.com" not "yourdomain.com". So check and change the variable if you got page not found error, then restart your stack, everything will be back to normal.