[NB] Can't write in documents,

if I work on a single document, I can use Scrivener normally. But if I try to open more documents in one editor-window (the documents in one folder), I can read them, but not write.
Is there are a button for locking, or so?

This is not working in the Beta yet, as mentioned in the Known Issues section at the Beta Page:

[url]Scrivener 3.0 Beta - Release Candidate 10 (Download Links & Change List)]

Known Issues
We are still implementing features and ironing out bugs, so you should expect to find things that aren’t working quite as intended. A few issues to be aware of in the current beta:

  • When inline or linked notations/footnotes are included in a selection and the selection does not span the entire paragraph, setting No Style (or Remove Style) will strip the linked notation and lose the annotation/footnote formatting of the inline notes unless all text in the selection had a character style applied.
  • Though we have the interface set up for the new compiler, it is not yet fully functional.
    - Documents in Scrivenings mode cannot currently be edited.
  • There is currently a display bug we are still working to fix whereby text in one editor may temporarily disappear after loading or adjusting the text display in a different editor. Resizing the editor will cause the text to reappear.
  • Project templates have not yet been updated for 3.0, so are not taking advantage of new features and may have some outdated content.

L&L is trying to get this implemented in the next Beta release (May 31): [url]When can we expect Scrivenings Mode to work? - #7 by MimeticMouton]