FiberError: Cannot switch fibers in current execution context

Problem/Motivation

Trying to run a manual backup I got
FiberError: Cannot switch fibers in current execution context in Fiber->start() (Zeile 50 in /web/core/lib/Drupal/Core/Session/AccessPolicyProcessor.php).

I can reproduce this and submitting the manual backup form shows the error page.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes