As reported by Volker on the dev list (hmm, WSoD on https://tiki.org/forum26 again by the way) "remember me" doesn't seem to have been working since the upgrades to 26.x, and he reckoned setting the pref session_storage=db
fixed it for him, so i've done that one dev only for now to see if there's a difference.
It logged me out, and presumably everyone else, sorry about that - could do with a warning 😬