Your LimeSurvey version: [6.0.0]
Own server or LimeSurvey hosting: [Own server]
Survey theme/template: [default]

Hi, we are experiencing issues after upgrading from 3.x to 6.0:

• Visit a public survey -> Click "Next" -> Receive the error "We are sorry but your session has expired."
• Visit the admin login form -> Log in -> Gets redirected to the login page after successful logins.

I suspect this is associated with our adoption of CloudFlare reverse proxy, but after turning off the CloudFlare proxy, the issue seems to persist. Has anyone experience the same issue? Any help is appreciated!

Issue with session not created or deleted.

Need more information on server settings (and look at your logs)
If you just upgrade : it's strange we don't update session system except we use a specific session name.