Thank you for your query. Could you provide more detail of your upgrade. What database, application server are you using? Which version did you upgrade from and to (7.1.x)? Are you using the correct browser versions?
Root cause:- Your application is still pointing to Pega lower version. (Like if you upgrade from pega 6.1 then your applicaiton must be point to PegaRules:06-XX-XX )
Solutions:- Log in with firstname.lastname@example.org and open your application and change Base rulesets version to 07-XX-XX eg PegaRules:07-XX-XX. Log out and login with your Application operator. This time it will work.