I’m running into a Major trouble.

Today I found out that when visiting a specific URL on my website, the admin toolbar shows up. Also for people who are not logged in. It appears that the visitor is logged in, but that’s not true. All the links in the toolbar go to a 404 page. However, important information is being shown, like how many plugins need an update and even my login name.

Is anyone familiar with this?

Since it is a corporate website, I need a fix as soon as possible. (due to security reasons I won’t post the exact url in public)