After upgrading one may see that the options window is not showing any options.
This is the same issue described in this post.
EPiServer.Marketing.Testing 2.5.5 has been released which fixes this issue: https://nuget.episerver.com/package/?id=EPiServer.Marketing.Testing&v=2.5.5
After upgrading, be sure to completely clear the browser resources. This includes clearing local storage, session storage and site cookies. In Chrome, the "Cookies and other site data" and "Cached images and files" options should do this.