The "Invalid Authenticity Token" error is usually caused by a problem with the cookies in your browser.
To troubleshoot this error, first try closing and restarting your browser to see if that fixes the problem.
If that does not work, please delete the Seamwork cookies from your browser. How you do this depends on which browser you're using. This page has instructions for doing this in all the major browsers:
https://www.beacontechnologies.com/blog/2012/10/clearing-cookies-from-just-one-site.aspx