I have seen this once or twice myself and if I remember correctly logging out and restarting browser fixed the problem, have you tried this? Also please let me know what version of Pega this happened on.
Cecil Howell, CSA | Senior Instructor | Pegasystems Inc.
Yes ... generally, you need to completely close the browser. If you have three instances of the browser running, then all three need to be closed. You don't need to go so far as to reboot the entire system.