After upgrading the extension it basically still works. However after confirming acceptance one gets redirected to Special:CreateAccount and you get a note saying "There seems to be a problem with your login session; this action has been canceled as a precaution against session hijacking. Please resubmit the form." The account however was created and the user may start using the wiki despite not submitting the form again. The site showing this behavior is: https://www.mediawikiwidgets.org/
Topic on Extension talk:UserAgreement
As a preliminary workaround I change the respective system message on MediaWiki:Sessionfailure to "Oops, redirection to the desired page somehow has failed. However, your account was created and you may now use the wiki!" :)