"The mass action is currently being run in another active request"
The system may return this error if you are working on multiple tabs. Normally, once you run a mass action, you should see a progress bar on top that will provide the status of the action you performed. The system will return an error if you try to run another action while the system is still performing the first action.
Most of the time, clearing your browser's cache/history and closing the tabs/windows, and logging back in will resolve this error.