Warning: Undefined array key 1 in Drupal\Core\Entity\EntityAccessCheck->access()

There is not much to go on there. Can you get us a full stack trace please?

As with the other issue we need steps to reproduce this and a full stack trace.

It is nice to see a patch but steps to reproduce are needed so that the problem is fully understood and a test can be written. Since that has not been supplied in 11 months, I am closing this issue.

Therefore, closing as cannot reproduce. If you are experiencing this problem on a supported version of Drupal reopen the issue, by setting the status to 'Active', and provide complete steps to reproduce the issue (starting from "Install Drupal core").

Just stumbled over this error message and I'm pretty sure I know the origin of it.

If you define a custom route which checks access like this:

entity.foo.bar:
  path: '/foo/{foo}/bar'
  defaults:
    _form: '\Drupal\module\Form\FooBarForm'
    _title: 'Foo Bar'
  requirements:
    _entity_access: 'foo.view'

In this case I realised that the user needed a different special permission than access to "foo.view" and I mindlessly replaced:

_entity_access: 'foo.view'

with:

_entity_access: 'my special permission'

.. but "my special permission" doesn't work in entity_access as the string is exploded at the "." and as the permission doesn't have a dot you'll get the error message above.

The correct way is to replace it with:

entity.foo.bar:
  path: '/foo/{foo}/bar'
  defaults:
    _form: '\Drupal\module\Form\FooBarForm'
    _title: 'Foo Bar'
  requirements:
    _permission: 'my special permission'

Tldr; if you see this error message thrown somewhere you should probably check your code base for routes using "_entity_access" with a string which doesn't contain dots.