“Platitudes” or not, it is a problem that was introduced with the most recent update to some part of Mojo or Mojolicious. Doing a manual installation of the prior version MojoX::Session (v0.07) apparently fixed the problem.

“I guess I was being a bit naive,” he admitted ruefully. “I guess I just thought that code which didn't work didn't make it to CPAN. And to be quite honest, it still seems strange... like I'm still missing something basic.”