If a form user clicks the "Sign the Section" button and is not already authenticated the form pops up a authentication light box where they can enter their username, password, tenant. If they do this they are getting this error: "The action could not be completed. This can happen if your session expires or if you are editing the same form/flow in multiple sessions".
This issue is logged as product ticket #13435. The work-around is for the user to login before accessing the form so that they are already authenticated before clicking "Sign this Section". You can force the user to login before accessing the form by making the form public in tenant .