Single sign-off: Logging out of the Dashboard doesn't log you out of Nextcloud (and probably also other apps)

Steps to reproduce (on stackspin.net):

• Login to the dashobard
• Click on "Launch App" in the NC box
• Another tab opens NC, you're logged in
• Go to the Dashboard tab, log out from there
• You get redirected to https://dashboard.stackspin.net/login, it seems like you're logged out of the Dashboard
• Switch to NC tab, reload the page
• You're still logged in NC