I was getting the above error "Your session has expired" immediately after upgrade of Oracle Apex from Version 5 to 20 in my premises. Searched a lot and did not find any resolution.
After trying to login in same browser, tried a private browsing mode in Chrome and Firefox. It logged in. That is when i realized that it might be due to session cache/cookie.
As a resolution I cleared all session cookies by clicking "Inspect Element/Storage" in Firefox. It worked after the storage for the site was cleared.
