Similar to the login to en.wikisource.beta.wmflabs.org is not possible at the moment. The login always fails with

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.

Seems to be similar to T243123: Login to at least en.wikipedia.beta.wmflabs.org and commons.wikimedia.beta.wmflabs.org sometimes fails with `There seems to be a problem with your login session` but happens all the time.