This error can be related due a stopped or frozen service of the WipeCenter.

Please make sure you have a terminal opened with the WipeCenter service running. This terminal may not be closed during operation. 

If the service is running and you keep getting this error, please stop the service and open FireFox and clear history.

You can do this by hitting CTRL + SHIFT + DEL and select the option 'everything' in the popup. Press OK, close FireFox and start service again. 

Now open FireFox and go to localhost:9000 again and log in. It should be solved. 

If the error remains, please contact and/or submit a ticket.