Strange behaviour on missing session

[kabarakh]

Description

When the Neos backend session times out (or the cookie is removed) and I click on a different document, I get three strange behaviours:

1. The Neos login is shown instead of the document I want to edit (which is fine), but this page is continuously refreshing.
2. I get a modal dialog to authenticate again (which is fine), but when I input my credentials it says "Wrong username or password". If I just go to /neos and use the same credentials the login works.
3. When I re-login after going to /neos (see step 2), after logging in I get to a preview-page without the Neos backend container, but I should get to the Neos backend

Steps to Reproduce

1. Login to Neos backend
2. Delete neos_session cookie
3. Select a document in the tree

Affected Versions

Neos: 5.1.4

[simonschaufi]

Can confirm this behavior. Related: #2779