Pega is authenticated on SSO. Issue is when the user logged in to application and keeps the system idle for more than the session time, it requires to close all the other IE browsers to re login!
Open some other IE session (any session beside PEGA session)
Login to Pega Dev or any environment via LDAP URL
Set your operator id and check external authentication check box checked
Login and go for coffee break
After 30 minutes, click on this session and it will log you out. Close this session and make sure you have other IE session open
Open new session and copy/paste URL or click it form favorites
It will give you error shown in following screen shot.
Only way, you can login is to close all open IE sessions then you can login.