Steps to reproduce the issue

Create new menu item of type 'Single Article' using 'Create' to create a new article
Save and close the item
Open the item again
Click on 'Edit' next to Article Name

Expected result

The article editor will open

Actual result

403 Forbidden error message.
Close, Save & Close and Save buttons do not work.
Escape does not work
The only way to get out of this is to close the whole tab

System information (as much as possible)

J4 Alpha 12 Nightly update 2019-09-20
PHP 7.2

Additional comments

Same result on Firefox and Chrome